because he's the only villain in the entire trilogy that when i found out about his motive, I went: "Oh, I can see why he's taking a shot at Tony."
"What your family did to my family over 40 years, I'll give it to you in 40 minutes.."
(i think that's what he told Tony on the phone)
from that point, i think his motive is the most understandable one while i'm not saying Stane & Killian are bad, but they had a similar "trying to get rid of my rival once and for all" kind of thing. Well, Hammer also had that motive but since he's so funny to watch especially a scene where he demonstrate the ex-wife, He's my second favorite