My faves are old stuff
Fantastic Four (esp. Byrne)
Excalibur (Chris Claremont and/or Alan Davis runs)
Captain Britain TPB is very good too, the new one with the classic Fury storyline should be coming out soon so be sure to pick it up! I've only read a little more than the second half but I can tell you it is an excellent story by Alan Moore w/art by Alan Davis!