Although I haven't finished this yet, some people might still find it useful in its current unfinished form, which is why I am making it visible to y'all, but not yet submitting it to be featured.

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Those of you who read my old blog about getting Pokémon with high stats (and those of you who don't need to) may be wondering where the best places are to EV train your Pokémon in different Pokémon games.

Although it is somewhat a matter of personal preference, some places are just better for working on certain stats than others, either because of multiple Pokémon in the same place that raise the same stat, or because of one extremely common Pokémon (in some cases the only Pokémon) in an area raising that stat.

Below are the locations and techniques that I tend to use for EV training in the Gen 3-8 games, and while I unfortunately can't yet provide any useful information about EV training in Sword/Shield since I don't yet have a Switch, some or all of the information about EV training locations in Gen 3 Kanto and Gen 4 Sinnoh may also apply to Let's Go Pikachu/Eevee's Kanto and Brilliant Diamond/Shining Pearl's Sinnoh respectively.
(They take place in the same regions, after all.)

[Spoiler=Gen 3 Hoenn (Ruby/Sapphire/Emerald)]EV training in Gen 3 is a PITN compared to more modern Pokémon games, not only because of the lack of "Power" items such as the Power Anklet, but also because of the lack of EV reducing Berries, meaning that all of the Pokémon that you played through the game with are inevitably going to have messed up EVs, and there is absolutely nothing that you can do about it.
(Aside from transferring them to Gen 4, anyway.)

As expensive as it is, IMO it is best to just blow the money on vitamin supplements such as Calcium and Protein to get your EVs to 100 before you start EV training in Gen 3, especially since it is impossible to earn more than 12 EVs per Pokémon you defeat.
(By defeating a Pokémon that gives 3 EVs with the 2× multipliers of both the Macho Brace and the Pokérus.)

Of course, if you know anyone IRL who also has copies of Ruby/Sapphire/Emerald (or have multiple Gen 3 Hoenn games yourself, and a 2nd GBA), you can take advantage of the Secret Base trick mentioned on this Serebii page to help make EV training significantly easier, although I suspect that most of y'all wouldn't actually be able to do that.
(I mean, how many people even still have GBAs?)

The following are the places where I used to go to train my Pokémon's EVs in Hoenn:

HP: Whismur in Rusturf Tunnel.

Attack: Poochyena on Route 101.

Defense: Geodude and Aron in Granite Cave, as well as the Geodude and Nosepass found by smashing rocks there.

Sp. Attack: Abra in Granite Cave B2; or Oddish on Route 110.

Sp. Defense: Tentacool and Tentacruel (gives 2 EVs) found by Surfing on most ocean routes in Hoenn.

Speed: Magikarp caught by using the Old Rod at almost any body of water in Hoenn. [/Spoiler]

[Spoiler=Gen 3 Kanto (FireRed/LeafGreen)]Like Gen 3 Hoenn, EV training in Gen 3 Kanto is a PITN (for the same reasons), and so it is best to get your EVs to 100 by using vitamin supplements before you actually start working on them IMO, which in turn means that you will probably want to use your Vs. Seeker to battle Trainers like person on place, in order to earn lots of money for buying them.
(And don't forget to use your Amulet Coin, as well as Pay Day.)

And of course, unlike in Hoenn, there isn't any Secret Base trick that you can use to give easy access to low level Pokémon that give 3 EVs, which means the best approach (IMO) is to simply look for common, low level wild Pokémon that give 1 or 2 EVs, such as those listed below:

HP: Nidoran♀ and Jigglypuff (gives 2 EVs) on Route 3.

Attack: Mankey on Route 22.

Defense: Geodude in Mt. Moon or Rock Tunnel; or Sandshrew (LeafGreen only) on Route 4.

Sp. Attack: Gastly and Haunter (gives 2 EVs) in Pokémon Tower; or Abra and Oddish (FireRed only) on Route 25.

Sp. Defense: Tentacool and Tentacruel (gives 2 EVs) found by Surfing on most ocean routes in Kanto.

Speed: Magikarp caught by using the Old Rod at almost any body of water in Kanto. [/Spoiler]

[Spoiler=Gen 4 Sinnoh (Diamond/Pearl/Platinum)]Gen 4 is when EV training FINALLY becomes easy enough and simple enough that a casual Pokémon player might actually find it worth bothering with.

By combining the effects of the Power items (Power Anklet, Power Bracer, Power Lens, etc.) with the effect of the Pokérus (which can now be acquired much more easily, due to online trading via Wiimmfi), you can earn as many as 10 EVs in a stat for defeating a single low level wild Pokémon that would normally give only 1, making the use of vitamin supplements (which also give 10 EVs) in EV training no longer worthwhile IMO.
(Especially considering how common the Pokémon that I list below are.)

Since there are plenty of places to grow Berries in Sinnoh, growing hundreds of EV reducing Berries such as Pomeg and Tamato shouldn't be that difficult to do, with a little time and effort.

Also, if you're trying to work on EVs for a team of baby Pokémon lower than Level 5 for use in Sunny Park Colosseum in Pokémon Battle Revolution, or Little Cup battles against friends, it is important to earn EVs against as low level of opponents as possible (which I always keep in mind with my recommendations), but it also helps to split Exp. Points between the Pokémon whose EVs you are working on and other Pokémon in your party.

The simplest way of doing this is to lead with the baby Pokémon whose EVs you are working on (who should be holding a Power item that gives EVs in the desired stat), and to immediately switch to a higher level Pokémon holding an Exp. Share, and to then have them defeat the wild Pokémon.

Doing this will cause the baby Pokémon to only get 25% Exp. (with the other 75% going to the higher level Pokémon with the Exp. Share), but it will still get just as many EVs, and since you are already getting 10 EVs per each wild Pokémon defeated (assuming that you are using both a Power item and the Pokérus), this makes it relatively fast and easy to max out the EVs of baby Pokémon without leveling them above 5, and without having to waste hundreds of thousands on vitamin supplements, either.

Speaking of which, if you're interested in earning money in Sinnoh, you can just repeatedly battle Gentleman Jeremy and Socialite Reina on Route 212 with your Vs. Seeker, and you can even work on your Attack and Sp. Attack EVs at the same time.
(Gentleman Jeremy's Chatot gives 2 Attack EVs, and Socialite Reina's Roselia gives 2 Sp. Attack EVs.)

Anyway, these are my preferred EV training locations in Sinnoh:

HP: Bidoof on Route 201.

Attack: Shinx* on Route 202; Machop on Route 207; or Gentleman Jeremy's Chatot (gives 2 EVs) on Route 212.

Defense: Geodude in Oreburgh Mine.

Sp. Attack: Gastly, Haunter (gives 2 EVs) and Gengar (gives 3 EVs) in the Old Chateau; Psyduck in Oreburgh Gate; or Socialite Reina's Roselia (gives 2 EVs) on Route 212.

Sp. Defense: Tentacool and Tentacruel (gives 2 EVs) found by Surfing on most ocean routes in Sinnoh.

Speed: Magikarp caught by using the Old Rod at almost any body of water in Sinnoh.

*Shinx is an Electric-type Pokémon, and you can dramatically increase the chance of running into them on Route 202 by having a Pokémon with the Ability Static leading your party, and then switching to whatever Pokémon in your party you are currently EV training.

†Haunter and Gengar only appear in the Old Chateau when a Gen 3 Pokémon game (Ruby, Sapphire, Emerald, FireRed, or LeafGreen) is inserted into the DS's GBA cartridge slot (which is absent in all DSis, 3DSs, and 2DSs) before you start playing, and the latter only appears in the room with the spooky painting in it.[/Spoiler]

[Spoiler=Gen 4 Johto (HeartGold/SoulSilver)]Just like in Gen 4 Sinnoh, EV training is pretty easy in Gen 4 Johto.

Although it is harder to grow massive numbers of Berries in Johto (due to only being able to grow up to 4 at a time in your Berry Pots), pretty much everything else I said about EV training in Gen 4 Sinnoh (above) still applies in the Johto remakes, including that Exp. splitting technique.

It is worth noting that by adjusting the DS's internal clock to be between 10:00 and 20:00 on Tuesday, you can repeatedly battle Gentleman Alfred in the Lighthouse, who not only pays out a large amount of money (which can be doubled by using an Amulet Coin), but whose Noctowl also gives 2 EVs in HP upon being defeated, making it possible to work on HP EVs and earn massive amounts of money at the same time.

To do this, simply call Alfred with your Pokégear between 10:00 and 20:00 on a Tuesday, battle him, step out of the room, and then call him again for another rematch.

This can be done as many times as you want so long as the DS's internal clock still says it is between 10:00 and 20:00 on a Tuesday, and unlike the Vs. Seeker in the Sinnoh games, Trainers called on the Pokégear will always want to battle you (provided that your DS's clock is set to the correct time of day and day of the week, anyway), and you do not have to repeatedly take 100 steps between rematches.

Below are my favorite EV training spots in Johto, many of which involve smashing rocks (which can also get you lots of items), headbutting trees, or fishing, as I tend to prefer doing those things over running around in the tall grass to find wild Pokémon to defeat:

HP: Hoothoot found by using Headbutt on trees in New Bark Town at any time of day, or in the tall grass of Route 29 at night; or Gentleman Alfred's Noctowl (gives 2 EVs), which can be repeatedly battled in the Lighthousebetween 10:00 and 20:00 on Tuesdays.

Attack: Sentret found on Route 29 during the day; Bellsprout and Ekans (SoulSilver only) on Route 32; or Krabby found by using Rock Smash in Cianwood City or Cliff Cave.

Defense: Pineco found by using Headbutt on trees in New Bark Town; or Geodude found in the tall grass on Route 46, or by using Rock Smash on most smashable rocks found throughout Johto.

Sp. Attack: Gastly found in Sprout Tower at night; Mareep* on Route 32; or Psyduck and Golduck (gives 2 EVs) found by Surfing on the pond on Route 35.

Sp. Defense: Tentacool and Tentacruel (gives 2 EVs) found by Surfing on most ocean routes in Johto; Hoppip on Route 33; or Drowzee on Route 34.

Speed: Magikarp caught by using the Old Rod at almost any body of water in Johto.

*Mareep is an Electric-type Pokémon, and you can dramatically increase the chance of running into them on Route 32 by having a Pokémon with the Ability Static leading your party, and then switching to whatever Pokémon in your party you are currently EV training.[/Spoiler]

[Spoiler=Gen 5 Unova (Black/White/Black 2/White 2)]EV training in Gen 5 is more or less the same as in Gen 4, other than that Level 100 Pokémon can now earn EVs, and that you can now increase EVs beyond 100 points without battling, by using Wings such as the Genius Wing, or (in Black 2/White 2) by taking them to certain shops in Join Avenue.

Also, it is worth noting that it is far harder to get those EV reducing Berries in Gen 5, making it much easier to work on EVs in the sequels, where you can not only purchase those Berries on Join Avenue, but can also reduce you Pokémon's EVs by having them groomed at a Salon.

You may need to reduce your Pokémom's friendship slightly before the Salon will be willing to groom them (if your friendship is maxed), which can easily be done by getting your Pokémon hurt slightly in battle and/or afflicted with a status condition (which a Toxic Orb, Flame Orb can do), and then using an Energy Powder or Heal Powder to heal them, right before you have them groomed at the Salon.

Also, it may be worth checking out this blog about EV training in Black/White, as they are good alternatives to the locations listed below:

HP: Munna in the Dreamyard (Black 1/White 1 only)†; Azurill on Floccessy Ranch (Black 2/White 2 only); or Audino (gives 2 EVs) found in rustling grass in most places throughout Unova.

Attack: Lillipup and Patrat on Route 1 (Black 1/White 1 only)†; or Patrat on Route 19 (Black 2/White 2 only).

Defense: Roggenrola in Wellspring Cave (Black 1/White 1 only)†; or Sewaddle and Venipede on Route 20 (Black 2/White 2 only).

Sp. Attack: Litwick and Elgyem in the Celestial Tower; Mareep* and Psyduck at Floccessy Ranch (Black 2/White 2 only); or Petilil (White 1/White 2 only) in Pinwheel Forest (White 1 only)†.

Sp. Defense: Frillish and Jellicent (gives 2 EVs) found by Surfing on most ocean routes in Unova.

Speed: Purrloin in the Dreamyard (Black 1/White 1 only)† or on Route 19 (Black 2/White 2 only); or Basculin (gives 2 EVs) found by Surfing or fishing with the Super Rod on most bodies of water throughout Unova.

*Mareep is an Electric-type Pokémon, and you can dramatically increase the chance of running into them at Floccessy Ranch by having a Pokémon with the Ability Static leading your party, and then switching to whatever Pokémon in your party you are currently EV training.

†The levels of the wild Pokémon found here in Black and White Versions 2 are WAY higher level than they are in the original Black and White Version games, making it not worth EV training here in the Version 2 games IMO.[/Spoiler]

[Spoiler=Gen 6 Kalos (X/Y)]Like I mentioned in my Getting Pokémon with High Stats blog, the fastest way (by far) to EV train your Pokémon is by defeating hordes of wild Pokémon which (if done properly) can make maxing out the EVs of all of your Pokémon ridiculously easy.

Let's suppose that you have 6 Pokémon whose Attack stat you wanted to max out, for example.

If your lead Pokémon is a high level Pokémon with a multi-target attack move (such as Surf or Earthquake), it can instantly wipe out an entire horde of Pokémon with 1 attack.

By turning on your Exp. Share, your lead Pokémon will share ½ of the Exp. Points (and all of the EVs) earned by defeating the opposing horde with the rest of the Pokémon in your party, which is then boosted by each of their EV training held items and/or their Pokérus.

If you defeat a horde of 5 Weepinbell (which give 2 EVs in Attack each) while all 6 of your party Pokémon are equipped with Power Bracers, and they have all been infected with the Pokérus, all 6 of the Pokémon in your party will receive a whopping 60 EVs in Attack at once, which means that by defeating 5 hordes of Weepinbell, you can max out the Attack EVs of all 6 Pokémon in your party simultaneously.

Even if you are fighting hordes of Pokémon that only give 1 EV each (which is usually the case), you will still earn 50 EVs in whatever stat for each horde you defeat (assuming you are also using the Power items and Pokérus, of course), and since you can combine this with the EV sharing effect of the Exp. Share, it makes the Gen 6 games the easiest ones to EV train your Pokémon in.
(You will need to spend a TON of BP on those Power items to be able to do this as fast as I am talking about, though.)

Another great way to work on your Pokémon's EVs in Gen 6 is Super Training.

While generally not as fast and efficient as defeating hordes, Super Training is a much more enjoyable and challenging way to work on your Pokémon's EVs, and it is (usually) still considerably faster than traditional EV training, especially if you are not using the Pokérus, the Power items, etc.

The Super Training screen also allows you to actually see your Pokémon's EVs, and if you are trying to EV train your Pokémon without leveling them up (for Little Cup battles against friends), Super Training is by far the best way of doing.

Also, for Trainers who wish to carefully manage their Pokémon's EVs (rather than just maxing out the 2 most important stats), Super Training offers a number of different tools that can help aid in doing so, such as the Reset Bag, which is not only the fastest way of getting rid of unwanted EVs (although it also wipes out EVs that you do want), but is also good for precisely measuring your Pokémon's current EVs.

To do this, simply save the game, and then have the Pokémon whose EVs you wish to measure hit the Reset Bag until it resets their EVs, and when it does so it will show exactly how many EVs they had in each stat, after which you can soft reset the game, now knowing exactly what your Pokémon's current EVs are, and which ones you need to increase, decrease, or ignore.
(Hardcore players like myself like to carefully manage our Pokémon's stats, although it is most certainly not necessary to do so.)

These EV training locations were chosen for the likelihood of running into a horde of Pokémon that increases the desired stat every time that Sweet Scent or Honey is used there, and while none of these places have a 100% chance of the correct kind(s) of Pokémon hordes showing up, the odds are always >50%:

HP: Hordes of Gulpin on Route 5.

Attack: Hordes of Weepinbell (gives 2 EVs each) or Arbok (gives 2 EVs each) on Route 19.

Defense: Hordes of Durant (gives 2 EVs each), Geodude or Aron in Terminus Cave.

Sp. Attack: Hordes of Vannilite or Smoochum in Frost Cavern.

Sp. Defense: Hordes of Hoppip on Route 7.

Speed: Hordes of Wingull or Taillow on Route 8. [/Spoiler]

[Spoiler=Gen 6 Hoenn (Ω Ruby/α Sapphire)]Just like in Gen 6 Kalos, horde battles are by far the fastest way of EV training your Pokémon, and Super Training is a great way of doing so without leveling.

The Secret Base trick from Gen 3 Hoenn technically still works, but it isn't really worth it IMO, although defeating a Secret Base Trainer with 3 Level 100 Arceus (and a Level thingy in their Base) can gives a MASSIVE amount of Exp. Points, especially when combined with the effects of Lucky Eggs, the Exp. Point Power, and the Exp. Share.

Although I am getting a bit off-topic here, if the Secret Base is set to Triple Battles, and you have a Pokémon in the center that is capable of 1HKOing all 3 opposing Pokémon simultaneously (such as a Level 100 Kyogre holding a Choice Specs using Water Spout), you can swap out the 2 side Pokémon for other low level Pokémon in your party that you wish to level, so that they get full Exp. Points, rather than the ½ Exp. that the Exp. Share would give them.
(Using Dark Void to put all 3 of your opponent's Pokémon to sleep can also be helpful for doing this sort of thing, BTW.)

Of course, if collaborating with a friend, you could simply make sure all 3 Arceus only know harmless status moves like Cosmic Power, and then you can temporarily teach one of your low level Pokémon a move like Toxic, and then just let Toxic wear down all 3 Arceus simultaneously, while you swap Pokémon out to ensure that everyone gets full Exp. Points from the battle.
(And if all of this seems impractical, always remember that you can turn off the Level Thingy if necessary, which will reduce the level of all of the Trainer's high level Pokémon in the Secret Base to Level 50.)

As for working on EVs in Secret Bases, I would recommend using the "do some exercise" skill that certain types of Secret Pals have, as it gives 30 EVs in a stat of your choice with each use, doesn't give any Exp. Points, can be used twice daily (once you have the Platinum Flag) per Secret Pal with the skill, all of which make it an easier (but slower) way of working on EVs than both horde battles and Super Training.

Anyway, these are the best EV training locations in Gen 6 Hoenn, and each of these hordes has a 100% chance of appearing when Sweet Scent or Honey is used at these particular locations:

HP: Hordes of [A=A=https://www.serebii.net/pokedex-xy/293.shtml]Whismur[/A] in Rusturf Tunnel.

Attack: Hordes of [A=A=https://www.serebii.net/pokedex-xy/353.shtml]Shuppet[/A] in Mt. Pyre.

Defense: Hordes of [A=A=https://www.serebii.net/pokedex-xy/027.shtml]Sandshrew[/A] on Route 111.

Sp. Attack: Hordes of [A=A=https://www.serebii.net/pokedex-xy/043.shtml]Oddish[/A] on Route 119.

Sp. Defense: Hordes of [A=A=https://www.serebii.net/pokedex-xy/333.shtml]Swablu[/A] on Route 115.

Speed: Hordes of [A=A=https://www.serebii.net/pokedex-xy/041.shtml]Zubat[/A] in Meteor Falls.[/Spoiler]

[Spoiler=Gen 7 Alola (Sun/Moon/Ultra Sun/Ultra Moon)]I am currently still working on my EV guide for the Gen 7 Alola games.[/Spoiler]

[Spoiler=Gen 7 Kanto (Let's Go Pikachu/Let's Go Eevee)]IDK very much about the Let's Go games, but the following locations are ones that I use for EV training in Pokémon FireRed Version, which takes place in the Kanto region just like the Let's Go games do:

HP: Nidoran♀ and Jigglypuff (gives 2 EVs) on Route 3.

Attack: Mankey on Route 22.

Defense: Geodude in Mt. Moon or Rock Tunnel; or Sandshrew (LeafGreen only) on Route 4.

Sp. Attack: Gastly and Haunter (gives 2 EVs) in Pokémon Tower; or Abra and Oddish (FireRed only) on Route 25.

Sp. Defense: Tentacool and Tentacruel (gives 2 EVs) found by Surfing on most ocean routes in Kanto.[/Spoiler]

[Spoiler=Gen 8 Galar (Sword/Shield)]I do not yet know enough about the Galar region or the Pokémon that inhabit it to recommend any specific locations for EV training.[/Spoiler]

[Spoiler=Gen 8 Sinnoh (Brilliant Diamond/Shining Pearl)]The following locations are the ones that I use for EV training in the Gen 4 Sinnoh games, and are more than likely good locations to work on EVs in the Gen 8 Sinnoh remakes, as well:

HP: Bidoof on Route 201.

Attack: Shinx* on Route 202; Machop on Route 207; or Gentleman Jeremy's Chatot (gives 2 EVs) on Route 212.

Defense: Geodude in Oreburgh Mine.

Sp. Attack: Gastly, Haunter (gives 2 EVs) and Gengar (gives 3 EVs) in the Old Chateau; Psyduck in Oreburgh Gate; or Socialite Reina's Roselia (gives 2 EVs) on Route 212.

Sp. Defense: Tentacool and Tentacruel (gives 2 EVs) found by Surfing on most ocean routes in Sinnoh.

Speed: Magikarp caught by using the Old Rod at almost any body of water in Sinnoh.

*Shinx is an Electric-type Pokémon, and you can dramatically increase the chance of running into them on Route 202 by having a Pokémon with the Ability Static leading your party, and then switching to whatever Pokémon in your party you are currently EV training.[/Spoiler]