The most hilarious movie I've ever seen.
Here are the 9 runners-up because they are worth mentioning:
Elf: You sit on a throne of lies
Raising Arizona: the chase scene
Ghostbusters: the stay-puft marhsmallow man
Airplane: Oh, stewardess, I speak jive.
Parenthood: Keanu Reeves should have stuck with this character.
Bananas: Woody Allen parking scene
A Fish Called Wanda: Don't call me stupid.
Hot Fuzz: One thing you haven't got
Clue: What, another door?
Galaxy Quest. Imagine actors, famous mostly, if not only, for their tenure on a sci-fi show, and who now make a living opening big box stores dressed in costume and attending conventions, try hard now, can you think of people like this? Well, then, imagine aliens coming down to earth and kidnapping them to take them to space--where the aliens have, based on the TV show they intercepted and thought was a "historical document", built a replica of the ship from the show. Thinking the actors are their characters, well, hilarity ensues. I especially love this movie because I was a hanger-on with a whole college crowd of sci-fi geeks. So some of this may be lost in translation. But this is my very favorite comedy. Ever.