This doesn't further the conversation at hand (which is how awesome Forgettig Sarah Marshall is), but Ben Affleck's best performances, in order:
1) The Town
2) extract (this was for Affleck what dazed and confused was for mcgoneghy)
3) good will hunting
4) mallrats (see #2, but less)
Also, since we is also discussing Paul Rudd movies, wet hot American summer is my favorite Rudd performance, and slides beyond Pootie tang for most underappreciated comedy