Personally I'd rate his teams in this order:
Sinnoh>Kalos>Kanto+OI=Hoenn>Johto>Unova
Kanto and Hoenn equal? Nostalgia much? Yeah, I'm aware Ash's Kanto team basically consisted of Pikachu, Bulbasaur, Pidgeotto and Charizard (Squirtle didn't do anything until OI). While Hoenn handled much better his team, there's Torkoal who received the shaft in everything and Glalie who appeared last minute only to battle and be left at Oak's right away. However, Pikachu's, Bulbasaur's and Charizard's character development throughout the OS remains one of the best in the entire series and redeem his otherwise badly unhandled team.
Then again, that's my opinion, so everyone's free to think different otherwise