...would have been Dirty Work if you asked me five years ago. I was unreasonably obsessed with that movie. Sure, the premise is pretty genius; a revenge for hire business. And since I was a Norm fan and Artie Lange wasn't yet the creepy guy on Howard Stern, I enjoyed the cast. Flawed? Certainly, but so little of it was to me that I would have verbally sparred with anyone who disagreed with my assertion that it was the greatest movie of all time.
Today? Jeez, I'm not sure. I saw Stand By Me a few times this year and I had forgotten how much that movie meant to me. Comedy wise I was obsessed with Let's Go To Prison for a while but it certainly wouldn't stand up against say, The Goonies. I can still watch Old School and laugh near constantly (although there are a few points in that movie that just are not funny). Forgetting Sarah Marshall was comic genius (for the most part - there were lame parts). I guess I'm a Stephen King fan because the other non-comedy I'd consider would be Shawshank Redemption; such a terrific movie.
I can not definitively say what my favorite movie of all time is although it would be no challenge at all for me to say what my favorite television show of all time is; Freaks and Geeks. There is no other show that is so flawless (even people I don't like (hey Seth Rogan - you aren't funny) end up being human by the end of the one season series. And don't even get me started on Bill, greatest character ever created in theatre; hands down.
So although I can't answer this question properly right now I figure that if you give me another couple years (at least - hopefully many more), I might have a better idea. The best movie ever made could be in post-production right now...you never know.