The second generation. It introduced many new concepts to the Pokemon series that have been incorporatedin many new games. Concepts such as held items, breeding, decoratible spaces, night and day, pokemon forms, shiny pokemon, traveling legendaries (entei, suicide, raiku, etc.), new kinds of pokeballs, and much more.
Second gen has some cool pokemon, but not too many with good stats. Also, second gen is the ONLY gen that has starters with pure typing with all their evolutions (meaning they arn't like fire/fighting, gras/poisen etc. Just fire grass and water). Fith gen has some dumb pokemon ideas, but alot of them have good stats. I'd go with first gen. It started it all. Also it had team rocket, the best enemy. Aqua/Magma Are real close though. Also, the first gen is the only gen that has a Rival thats actually challenging. And his oh so nice catch phrase "Smell you later" with his overpowered pokemanz.
I dream of a better tomorrow, where chickens can cross the road and not be questioned about their motives.