What is concrete is that Arceus has the highest total base stats. Which just means it has the greatest...potential. There is no strongest Pokémon because of various strengths and weaknesses due to varying natures, types, etc. This is a trend in games involving customisation.
By the formal definition of "epic", it's still debatable. I go by legendaries because of simple rarity and they expectedly carry the most heft. Starter evolution chains aren't in my book because of availability. Mewtwo is the most widely known in the real world, but its artificial origins diminish the status of renown. In-game is where it gets very hairy (npi), as it can go either way in terms of physical presence or word of mouth. On a whim, I go with Entei. The average person won't know what a Darkrai is. Still; possibly the single greatest legend created by Game Freak remains its most infamous.
I would think, by my opinion, that any Pokémon can be the strongest depending on it's move set and nature. By my preference, I would think it would be Houndoom and Lucario.
My Blazekin on Pokemon Sapphire beat the elite four 21 times solo, easily. 1-2 hit killed every one of them, one move. He knows blaze kick, double kick, sky uppercut, and return (max powerered return)
I don't know exactly but i'm pretty sure he's one of my best pokemon.