Gonna have to say Lance on this one. He was stronger in RBY but wasn't the champion then technically. As far as raw numbers go, he had the weakest team in GSC.. however the level curve meant he was more than enough of a challenge for the player so.. its a tough call.
Wallace is a close second.
"pkmn X focused on the online, making it easier to trade and battle. but this will never work unless they remove this horrible metagaming." - immadbro