One scene or line from any movie...

[Kirby]

National Lampoon's Vacation when they're driving through the rainstorm with Aunt Enda on top of the car, still clutching her purse.

Best in Show - Every scene with Fred Willard, but especially the scene when he's talking to the other commentator about dogs and then says "a little off topic, but how much do you think I can benchpress?"

or when the two gay guys are packing for the trip:
"You packed 8 kimonos and we're only going to be gone 2 days."
"yeah, one more..."

 

[Mr. Laser Beam]

The scene in Airplane II when the flight crew members meet for the first time. Dunn, Under, Oveur

And of course the immortal William Shatner Scene:

Murdock: We'd better get to the tower, Lieutenant.
Lieutenant: We have no tower, sir.
Murdock (shocked): No tower?
Lieutenant: Just a bridge, sir.
Murdock: WHY THE HELL AREN'T I NOTIFIED ABOUT THESE THINGS?!?!?

 

[captcalhoun]

http://www.youtube.com/watch?v=7bKfSVjXs0Y

 

[Temis the Vorta]

"Gentlemen, gentlemen! You can't fight in here! This is the War Room!"

"Son, you got a panty on yore head."

 

[gblews]

"Okay, you take the blonde and I'll take the one in the turban". -Igor, "Young Frankenstein"

 

[Mr. Laser Beam]

"And a DOUBLE dumb ass on you!" - Star Trek IV

 

[Ethros]

From Encino Man (aka California Man)

Wheez the juice!
"We figured we'd wheez a little ju-uice"
"No wheezing the ju-uice"
"Wheez the juice!"
"No, no! No wheezing the juice!"

 

[Timby]

http://www.youtube.com/watch?v=ANvKkdkZm2o

 

[Nerdius Maximus]

http://www.youtube.com/watch?v=MjWFZPJZTxU
It's not too often that you get to see grown men beating up children in a movie.

 

[CommodoreKong]

"Back off man, I'm a scientist"

"What are you, some kind of cosmonaut?"
"No, we're exterminators. Someone saw a cockroach up on twelfth."
"That's gotta be some cockroach."
"Bite your head off, man."

"Ray if someone asks you if you're a God you say YES!"

There are so many more great lines and scenes in Ghostbusters.

 

[Misfit Toy]

"Excuse me while I whip this out..."

 

[The Boy Who Cried Worf]

http://www.youtube.com/watch?v=MjWFZPJZTxU
It's not too often that you get to see grown men beating up children in a movie.

The ultimate modern-day wish-fulfillment fantasy.

 

[tharpdevenport]

"Clue"

"Will you stop doing that?!"
"No!"

Just the way the butler says it.

 

[PKTrekGirl]

No one beats William Powell and Myrna Loy in The Thin Man flicks for more hilarious quotes per foot of film.

But the best:

Nick Charles: I'm a hero. I was shot twice in the Tribune.
Nora Charles: I read where you were shot 5 times in the tabloids.
Nick Charles: It's not true. He didn't come anywhere near my tabloids.

Runner up:

Reporter: Say listen, is he working on a case?
Nora Charles: Yes, he is.
Reporter: What case?
Nora Charles: A case of scotch. Pitch in and help him.

I love William Powell and Myrna Loy.

 

[Kelso]

I love William Powell and Myrna Loy.

You have exceptional taste.

 

[CommodoreKong]

"Clue"

"Will you stop doing that?!"
"No!"

Just the way the butler says it.

This man is drunk. Dead drunk.

Well to make a long story short...
Too late

 

[shivkala]

"Gentlemen, gentlemen! You can't fight in here! This is the War Room!"

*ahem*

I also want to add: Oops

and

A classic conversation

 

[Kira's Mom]

Just thought of one more- from The Goonies, Chunk confesses to the Fratellis.

But the worst thing I ever done...

By the time he gets to the last "HUUUU-AAAAHHHHHH!", I'm on the floor laughing.

 

[Miss Chicken]

I like almost every scene from Monty Python and the Holy Grail but I think that these are my favourite lines.

DENNIS:
Listen. Strange women lying in ponds distributing swords is no
basis for a system of government. Supreme executive power
derives from a mandate from the masses, not from some farcical
aquatic ceremony.
ARTHUR:
Be quiet!
DENNIS:
Well, but you can't expect to wield supreme executive power
just 'cause some watery tart threw a sword at you!
ARTHUR:
Shut up!
DENNIS:
I mean, if I went 'round saying I was an emperor just because
some moistened bint had lobbed a scimitar at me, they'd put me
away!
ARTHUR:
Shut up, will you? Shut up!
DENNIS:
Ah, now we see the violence inherent in the system.
ARTHUR:
Shut up!
DENNIS:
Oh! Come and see the violence inherent in the system! Help!
Help! I'm being repressed!

 

[Orac Zen]

This. I watch the movie at least once a year and when that scene rolls around I start laughing straight away.

Claudette Colbert hitchhiking, It Happened One Night.

The scene in Sense and Sensibility when Lucy reveals to Fanny who it is she plans to marry. Fanny's reaction makes me laugh until I hurt.

There are others, but that'll do for now.