I'm sure this has been done before but I think a major reason that Zuko's narrative arc in books 2+3 is so universally acclaimed is that it's essentially just a version of the monomyth wherein the "ultimate boon" is the ability to let go of selfish rage and live empathetically.

Was thinking the other day about how Aang's path to becoming the avatar works perfectly in the context of the archetypal hero's journey (right down to the 17 stages outlined by Campbell) and subsequently realized the same thing is true of Zuko in books 2+3,

leading me to the conclusion that having two monomyth stories occurring simultaneously, their heroes beginning on opposite sides of the same conflict before reconciling and reaching apotheosis at the same point is a key part of what makes the climax of book 3 so satisfying