These are pretty much excerpts from my all-time top 10...
Ghostbusters -- I've seen this hundreds of times, and I still notice new things or laugh at something I didn't notice before. This is one of those films where I don't feel like any scene was wasted. The entire cast rocks, and although he doesn't have too many lines Ernie Hudson has some great ones like, "That's a big Twinkie," or, "I have seen *shit* that'll turn you *white*!"
This is Spinal Tap -- This'll never get old for me. When I first saw this, my buddies and I sat there laughing, but there were times where we were looking at each other wondering, "Is this for real?" So many subtle and not-so-subtle references to *everything* rock-related, and was one of many excellent Rob Reiner films (The Princess Bride, Stand By Me, When Harry Met Sally..., etc.).
Singin' in the Rain -- Technically a musical, but there are so many funny moments in this, I think I can count it here. Donald O'Connor really holds his own with Gene Kelly, and Debbie Reynolds sparkles with youthful enthusiasm and excellent comedic timing.
The Big Lebowski -- I love every scene in this film, and some of the funniest exchanges and lines take a few viewings to catch...
Walter: "Does he still write?"
Pilar: "Oh no... he has health problems!" (guy's in an IRON LUNG)
Walter: "Say what you will about the tenets of National Socialism, Dude... at least it's an ethos."
The Dude: "Yeah." (sitting at a bowling alley bar eating peanuts)
Glengarry Glen Ross -- Not a comedy first and foremost, but the dialogue sparkles so much, and really, does it get any better than hearing Jack Lemmon say c***sucker? Most of the scenes in this film are funny as hell, and is endlessly quotable.