Let me put it in perspective for you guys-
Charizard gets frozen= fails the fight instantly
Pikachu gets frozen= instantly breaks out and then wins against a legendary
It makes no sense for Charizard to be his best. Sure Charizard is definitely one of his strongest, but THE strongest? No way.
But who cares? Oshawott the king beats both of them.
Edit: Granted, Charizard did fail to break the ice before he started his intense training at the valley, but he's a fire type. No excuse.