Top 10 Avatar The Last Airbender Characters*
1. Uncle Iroh (Very much the "Yoda" of the show, but not for the main hero.)
2. Zuko (A great character arc)
3. Aang (A classic main hero with an exciting and fun character progression.)
4. Sokka (Keeps you laughing every episode. Nice development as a strategist and fighter.)
5. Azula (A cunning psychopathic villain who poses a more persistent threat than Fire Lord.)
6. Appa (Kind of Chewbacca and the Millennium Falcon rolled into one. Such an loyal and endearing companion.)
7. Katara (Solid character and power growth with a great finale battle. She feels a little one-note at times, but she has some interesting tangent stories.)
8. Toph (Such a cool, and well animated, intro episode. A tough and talented character. Maybe not as much character growth, but she softens a little.)
9. Fire Lord Ozai (He's not really "active" until later in the show, but is a looming menace. The bits you do see prove that he's a bastard.)
10. Jet (A cool side character with a compelling, if a bit twisted, motivation. He does have an impact on Team Avatar both early and later in the show.)
Honorable mentions: Combustion Man, Avatar Roku, Momo, Ty Lee, King Bumi, Master Piandao, Long Feng, Monk Gyatso
I'm not sure about Jet, but I do like the character. Maybe Roku or Combustion Man could have made the last slot.
*Coincidentally, I am currently re-watching the show again. So good.