I love a good parody …
Blazing Saddles, Airplane!, Austin Powers (I and II), and, of course The Naked Gun (all three of 'em).
Beetlejuice
Monty Python and the Holy Grail
Pee-Wee’s Big Adventure
UHF
The Producers - the original, with Zero Mostel and Gene Wilder.
Raising Arizona, O Brother Where art Thou? and Joe Dirt all three crack me up every time I see them.
Usually the original French “The Visitors” (subtitled) will crack me up too, but I have to go put it in the DVD player, so I’m expecting it.
Office Space
The Princess Bride
Undercover Blues (My name is Muerte!)
What About Bob?
Ghostbusters
Groundhog Day (Pretty much anything will Bill Murray in it, actually)
Airplane
Clerks
Dogma
That’s about all I can think of.
The second half has its moments, though:
“Sir, does this mean Ann-Margaret’s not coming?”
“Easy. You don’t lead 'em so much…Ain’t war hell!”
“Well, pilgrim, only after you eat the peanuts out of my sheeeiiit.”
American Psycho.
What?
Another vote for Meet the Parents. Plus There’s Something about Mary. Although this may have to do with the fact that I’ve never seen it from start to finish.
And Annie Hall - the intellectual in the film queue (and Allen’s insecure reaction) gets me every time.
Dumb and Dumber
Coming To America
Freddy Got Fingered
Come to mind right away
I had the exact same reaction to Zoolander (and Meet The Parents).
Pretty much anything Monty Python
A Fish Called Wanda
Office Space
Snatch
Actually most of the movies people have mentioned crack me up. So I guess I’ll just make it easy and say E. All of the above. 
Special mention to The Emperor’s New Groove. That movie always has me in stitches.