Qwizmaster
Pokemon Trainer
Honestly? I think Kanto is not only not the strongest, but the weakest team Ash ever had.
Sure Charizard, Squirtle, and Bulbasaur may be powerhouses now but back in Kanto most of Ash's badges came from pity. The rest of Ash's team was either shafted (Pidgeoto), released too early(Butterfree and Primeape), or were left to rot at Oak's lab.
If we're judging based on how strong they are now, then the Kanto team is the strongest (even though Pikachu, Squirtle, Bulbasaur, and Charizard are the only remaining ones who can be considered part of the "main team"). However, if we're basing this on the teams performance during their respective region:
From strongest to weakest:
1. Sinnoh
2. Hoeen
3. Johto
4. Kanto
Very well put