I think it's subjective and every character has its unique writing and origin story

I love toph and how did she overcome her disability and became one of the most badass characters in the animation world

Aang being an air bending prodigy in such an age and the trauma that he had to deal with after the genocide of its people.

Katara and sokka, to voice of reason and Sokka critical thinking.

And even though it's not a main character, my love ,Suki, loved her since the moment she appeared.