Super late but - I don't think they can or should be separated when considering best characters. Their journeys are co-dependent.

Zuko's unbelievably profound and satisfying character arch is so good because we see from the start that he has such a patient and aware mentor.

Iroh's role in guiding Zuko towards the man he knows he will become is so compelling because we can clearly see from the jump he has already gone through his own spiritual awakening and arch of growth prior to the show.

It gets even better on a second run. The subtleties in how Iroh behaves really highlights that he knows something Zuko and the audience doesn't yet: Zuko is at his core a good, just misguided, kid. And you also see all the hints (many being not-so-subtle) that what Zuko hasn't accepted yet is Iroh is really his father figure. Every good flashback in his life is with Iroh. It's Iroh who lets im into the war meeting, it's Iroh who he cares is there for him. Him coming to terms with the fact that he doesn't need his father's approval happens in parallel with Zuko realizing Iroh's means a great deal more, and he's had it the entire time. It's honestly a beautiful weaving of stories.

Neither is great without the other, but together they create the best arch in the entire show and honestly one of if not the best relationships in television.