For me, Hulk vs. Abomination. Two huge freaks punching each other and destroying everything on their path. I loved it. CGI wasn't perfect but I didn't care much about that at that point. Nice reminder for me to get that film on blu-ray. Just for the final fight, it's worth it.
About the Avengers...I like the movie and the final battle. The enemies are just so bland that I couldn't get excited as much as I wanted. It all just rolls along conveniently and cool stuff happens, but that's it. Some characters during the fight (Natasha) were a little...I don't know, don't wanna say bad but something was missing. Still a very good long battle and I'm glad Hulk was great in that one too, but it could've been so much better. Still probaly 2nd for me.
IM1 would be 3rd for me. Easily the best 1-on-1 fight of all Iron Man movies. Counting Avengers' forest fight with Thor. Iron Man really needs another good 1-on-1 fight in the upcoming films. When they are good, they are really good.