I had an interesting idea. What if you took Generations I and II and pitted them in battle against Generations III and IV? I'm not just talking Pokémon, either. I mean trainers, Gym Leaders, Elite Four members, and evil teams. Who would win in what would undoubtedly be an earth shattering war?
EDIT-Since Ryuutakeshi pointed it out, let's also assume that it was only the Pokémon from each generation fighting. So only The Hoenn and Sinnoh Pokémon would be fighting for that generation.
I personally am throwing my vote to Generations III and IV because they are larger regions with more diverse Pokémon. Sorry, I and II...
I left Isshu and Gen V out because I haven't played it and is therefore irrelevant.
(note, the remakes Pokémon FireRed, Pokémon LeafGreen, Pokémon HeartGold and Pokémon SoulSilver would be with Generations I and II)