You're absolutely right that there couldn't have been a better team, but you're wrong in that there could have been a better team. Toph is infact the best earthbender at the time, but she cannot or does not lava bend. She barely grasps sand bending for the longest time only mentioning it again in the sand castle contest in the second to last episode. Zuko never created his own lightning which would have made the Azula fight turn on its head if the two had a Sozin Comet Lightning Agni Kai. He also can't combustion bend or stop it by any means of counter fire bending iirc. Sokka was the best that a non-bender can be. That being said he could have been any bender and still infinitely more helpful. Yeah Katara is as good as it gets besides having the blood bending Yakon gene