The only ATLA characters I've tried playing are Azula and Ty Lee, and while they're both very fun, I'd have to say Azula is my favorite- mostly because of the psychological aspect. I'm OBSESSED with psychology (sociopathy in particular, and since I see Azula as a sociopath... yay?), and it's fascinating to explore the different facets of her personality. As for Ty Lee, I've only been playing her in an actual *game* for a few weeks, but her UBER-FRIENDLINESS is just awesome to play out- she's much more relaxing than Azula, for obvious reasons. ;D
... Anyone else? :D?