aang, for several reasons.
1) we got very little backstory on most of the other avatars with the exception of roku
2) I find korra to be exceptionally one dimensional, she didn't get a lot of actual development and while I understand that every avatar is their own person, she was the stereotypical, it's all about me and im'ma kick your *** cuz I'm the avatar. If I want a female character like that, I'll stick with Toph or Azula, they had experience, they could back their words and they had more character development then I'm an all powerful bad-***. touches of humanity, such as when toph and sokka had a heart to heart in the runaway episode, or when azula acted like a sister when zuko returned from exile and they went to ember island.
3) aang was himself at heart, a happy go lucky kid, but he could be serious, he could be spiritual, he could be realistically emotional and fierce too when he was absolutely forced to it.
I would however love episodes that showed some of the other avatars lives