Spinal Tap, Raising Arizona, Airplane!, Animal House, Roxanne.

Airplane!, Office Space, Napoleon Dynamite, Kentucky Fried Movie, Blazing Saddles

Honorable Mentions: Austin Powers, Happy Gilmore, Royal Tenenbaums, Wayne's World, Dumb & Dumber

Not-really-comedies that are hilarious: Fargo, Pulp Fiction

I love Frances McDormand in that one.

It Happened One Night
The Lady Eve
Some Like It Hot
Blazing Saddles
Airplane!
S.O.B.
Singing In The Rain
This Is Spinal Tap
The Jerk
Bull Durham

Idiocracy
Midnight Run
Office Space

Lots of the ones here already are great.

Ace Ventura

Tootsie

While Shaun of the Dead is also good, I prefer Hot Fuzz.

All Peter Sellers movies as well.

Philadelphia Story
Get Shorty
Caddyshack
Animal House
It Happened One Night

Top-of-mind random list. There are more. Lots more.

It's tough a call to single out my favorite but Gene Wilder and Zero Mostel together can't be matched, IMHO. I like all the other Mel Brooks comedies, especially Young Frankenstein, the Marx Brothers, especially The Cocoanuts and Monkey Business, Woody Allen's comedies, especially Broadway Danny Rose and Zelig, as well as Arsenic and Old Lace, The Pink Panther, Being There, and Dr. Strangelove.

Tom Cruise's standout performance makes it a modern comedy classic.

Kung Fu Hustle is hilarious in a unique Hong Kong way.

Dumb and Dumber and The Mask are guilty pleasures.