I would say Toph, but I dislike how stagnant her character is. She is nearly the same person we meet in The Blind Bandit. In terms of each charecter on their own, I would say Toph.
However, the interaction between Zukko and Iroh is some of the best writing I've seen, and it was kept me watching the show at a point. Their progression and the insights into human relations they provide is simply stunning. So stunning in fact, I was talking to an Ex a few nights ago who asked me if I hated her for breaking up with me because she couldn't bare with a long distance relationship. I told her no, and then told her the story of Zukko and Uncle, how Zukko left Uncle in a prison, and when he met him again, expecting his uncle to hate him, but he was proud of him.
Though she had never seen the show, this helped her understand exactly where I was coming from, as well as helping her see that she truly made the right decision in ending us. I can't think of many shows where I can do that with.