1. Zuko- Extremely misunderstood and tortured dude. I have never seen him as a villain at all. Unlike Zhao and Azula, his quest to capture Aang was his quest for acceptance and compassion. I really like Zuko and I feel unending sympathy for the poor guy; usually you don't feel for the villain...He's much more of a tragic character. In my eyes, Zuko is more of a victim than even Aang.
2. Iroh- Like others have said, he was never really shown to be anymore than a good guy. I think it's quite obvious that he never believed in capturing the Avatar, he just encouraged it for Zuko's sake.
3. Zhao- Yep.
4. Fire Sages- Okay
5. Faceless Soldier- WTH?
6. Jet- Yes what he was doing was pretty sick and twisted, but he redeemed himself.
7. Jun- Redeemed
8. Koh- Actually, not much of a villain...
9. Azula- Hell yeah!
10. Mai- Redeemed
11. Ty Lee- Redeemed
12. Xin Fu- Sure.
13. Master Yu- 'Kay.
14. Dai Li- Damn sure
15. Long Feng- Great villain in "City of Walls and Secrets" and "Lake Laogai", but became pretty weak afterwards. He desreved better.
16. Qin- I think for the most part, he was there for a little something we all like to call comic relief.
17. Sandbending Dude- I don't know if you can really call him a villain. That's kind of like calling Yon Rha a villain. It was a misunderstanding.
18. Ozai- F**k yeah! Luke Skywalker's gone evil! Evil I tell you!
19. CM- Yes, and he sucked. They couldn't kill him off soon enough.
20. Sozin- Yeah.
21. Azulon- Yep, that f***er is pretty twisted. Ordering his 10 year old grandson to be murdered...How cruel and vile can you get?! Oh wait, I'll just go and ask that question to Ozai and Azula.
I didn't really make it past the 10th post here, but you left out a couple. You left out both Wardens, General Mung and Hama.