For me it’s these:
“One, Two, Three”
“What’s Up Doc” [& I’m not even a fan of Ryan O’Neal or B. Streisand]
“A Night at the Opera”
“Return of the Pink Panther” [that’s the one with Christopher Plummer as the jewel thief]
So what movies always make you laugh?
The Princess Bride
Office Space
Tommy Boy
That’s about it
')
I second Office Space
Also, Dumb & Dumber just kills me everytime I see it. I know it’s juvenille, offensive, and well, dumb, but there you go.
I can’t help it, but Zoolander has made me laugh every time I’ve watched it (except the first time; for some reason I thought it was intolerably dumb). Also, Kung Pow: Enter the Fist, but only if I watch the first hour or so and stop before the really bizarre and kind of not funny ending occurs.
Office Space and The Jerk .
Ace Ventura
Office Space
Meet the Parents
Austin Powers
Office Space
Weekend At Bernie’s
Innerspace
Six replies so far and nobody’s mentioned The Big Lebowski yet? You people should be ashamed of yourselves.
I second What’s Up Doc. Also,
Bedazzled (the first one, of course)
Dr. Stangelove
Sleeper
MST3K the Movie
I never fail to crack up at the first half of Full Metal Jacket. The second half? Not so much.
I may be alone, but when I’m in a bad mood, there’s nothing that cheers me up faster than David Alan Grier playing Don “No Soul” Simmons in Amazon Women on the Moon.
I just chuckled while typing this, too. 
Also, apropos of nothing, any movie featuring monkeys in people clothes is also good for a smile.
My love of Hudson Hawk is clearly the reason that my application to the Cool Kids’ Club is rejected year after year. It was on USA the other day, and the bastards cut out all the best yuks. They allowed James Coburn’s “I’ve always had a soft spot for Rome . . . " but inexplicably did not include the follow up. (”. . . I did my first bare-handed strangulation here.") Also, they took out the Pope banging on television set with his . . . uh . . . whatchamacallit . . . his curly Pope-staff thing. What is up with that? I can only assume that it was cut for time, 'cause that, friends and neighbors, is totally inoffensive comedy gold. I challenge even the most uptight Catholic not to snicker at that. (“Catholic girls are scary!”)
Dumb and Dumber
What About Bob?
The Princess Bride
and the scene in Prisoner of Azkaban where Harry is sitting on the rock trying to convince me that he’s incredibly furious at the betrayal of Sirius Black and he makes this funny crooked mouth that he makes to show anger. I feel guilty for laughing at that scene when it’s supposed to be so moving, but it’s hysterical!
The Party and I Love You, Alice. B. Toklas, for sure.
Beyond the Valley of the Dolls.
Harvey.
The Forbidden Zone
Mr. Blandings Builds His Dream House.
Some Like It Hot
The Great Race
Young Frankenstein
The Great Dictator
Just about all of John Hughes’ movies make me laugh, such as:
Planes, Trains and Automobiles
National Lampoon’s Vacation
National Lampoon’s Christmas Vacation (especially the plastic lawn reindeer-kicking scene)
Ferris Bueller’s Day Off
The Breakfast Club
I’d also nominate Office Space along with several other people here. *What About Bob?[/], Short Time, South Park: Bigger, Longer and Uncut are also on my list.
Brain Donors
The Emperor’s New Groove
Galaxy Quest
South Park: Bigger, Longer and Uncut
Airplane
Rat Race
and, as plenty of others mentioned, Office Space.
Strange Brew
**Harvey
Airplane**
My all-time funniest movie:
A Funny Thing Happened on the Way to the Forum