Disclaimer - Avatar: The Last Airbender and all related characters belong to people that aren't me, no matter how hard I wish they were mine.
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When Sokka looks at his friends he sees them for what they are. The Best.
Katara, as much as she loathes and fears it, has a power only one other possesses. She is, without a doubt, the most powerful water bender in the world, but she'd never admit it. As far as Sokka's concerned, that's fine. He knows that it isn't power that makes someone great, but temperance. Just because you can do something doesn't mean you should.
Aang is the best air bender in the world by sole virtue of being the only air bender in the world. Even if a dust devil was the extent of his abilities wouldn't change that fact, but Sokka likes to think that even at the height of the Air Nomads' glory, Aang would have been something special.
Toph is something else altogether; brash and cocky to the point of arrogance, she knows she's the best earth bender alive and would have no problem letting the world know it too. If they make it out of this alive, Sokka plans to help her spread the news.