Spoilers will be below.
For me, I've gotta say 'Fire.' While it's missing the best character the show's ever had (Iroh) for a lot of it we finally get to see Zuko redeemed and joining the Gaang just like we always wanted. The biggest thing that I ever found frustrating was that I wanted that redemption arc for most of the Slytherins in Harry Potter. Draco seemed like the perfect person to get a redemption arc for the entire book series, but while it's obvious by the end that Draco is a complicated person who finds himself way over his head, he doesn't ever officially become a 'good' character.
So what about you? What is your favorite Book arc?
Tomorrow: Favorite Weapon