I've always been interested in Under Siege, but lately I've really been curious about Acts Of Vengeance. I know it was a company wide crossover, but it was mainly an Avengers event. I found one issue of Captain America that tied in and bought it because it had Magneto Vs Red Skull, and it was awsome. So now I want more.
As for my favorates. My favorate classicy run was the 90's run when Proctor was the main villain. That was the only era of Avengers pre-Bendis that could keep my attention. I've even had it bound.
The 2 Gray Volumes:
Beyond that I've loved Disassembled to current. I liked the original team of New Avengers best. I haven't, however, been a big fan of Mighty Avengers, neither under Bendis or Slott. Just hasn't been doing it for me.