What is the single funniest movie you've ever seen?

Oh, worth mentioning re: The Trouble with Lou. The producer is Teddy Newton, who’s now at Pixar and directed the Night and Day short. And the lead, Lou Romano, is an Art Director at Pixar and played the voice of the novice chef Linguini in Ratatouille.

For those who liked Noises Off:

My “whole cinema laughing like ticklish hyenas” movie is Japanese farce Welcome Back Mr McDonald. I saw it while visiting a friend in Kuala Lumpur, and went in somewhat nervous about a subtitled Japanese movie about a radio station.

It was hilarious. Brilliantly conceived, steadily escalating farce. As I didn’t have a clue what it was about going in, I’ll put this in spoilers for anyone who’s willing to trust me:

[spoiler]The set up is that the station is about to broadcast - live - a play written by a quiet unassuming housewife. The play is about the inner life of a quiet unassuming housewife. As written, anyhow. But the actors decide that this is way too boring, and start changing details on the fly: the lead character becomes a high powered trial lawyer; her salaryman husband becomes an astronaut; there’s a murder and a torrid love affair.

From the point of the first improvisation, various factions in the studio emerge:
The actors, all fighting for the biggest, most exciting part;
The sound effects guy, desperately trying to keep up with the new demands (e.g. a rocket launch) that weren’t really covered by his prep for a domestic mediation;
The author/housewife, trying to keep some semblance of her story intact;
Her husband, who can just about make out the original story and doesn’t like it;
The producer, who didn’t like the original play but is damned if his actors are going to win this power game.

As this is all live, there’s a lot of manic panic and physical comedy and the timing of each disaster, revelation and plot twist is superb throughout.[/spoiler]

I don’t know if it’s possible to get in the US, but if you like farce it’s well worth the effort.

I just wanted to observe that A Fish Called Wanda did absolutely nothing for me. Nor did Caddyshack. I’m not putting down people who liked these. Different Strokes, De Gustibus, and all.
Arsenic and Old Lace is one of my favorite movies (and stage plays). It’s beautifully constructed – the way people move in and out of scenes is balletic. And the references are sly. (By the way, if you you’ve only seen the movie, you owe it to yourself to read or see the stage play. They could get away with more on stage than the Hays office would let them do on screen. And it ends differently).

By Arsenic and Old Lace doesn’t have me laughing out loud the wat What’s Up Doc? or the others I mentioned do.

The film I probably laughed the hardest at while watching it for the first time was Flushed Away, although admittedly it was made funnier by being in a cinema full of ten-year-olds who didn’t get any of the in-jokes and cultural references. Screw you, kids; I thiought it was hilarious.

But funniest through repeated viewings? Yeah, probably The General or that one Pink Panther movie (the one with all the foreign assassins and the [del]1920s style[/del] death ray).

I also want to mention UHF. It’s not the funniest comedy ever but dammit, it’s underrated and needs some love.

And to the OP: I’d go with Rejected over Billy’s Balloon. I still laugh at “My spoon is too big!” and “MY ANUS IS BLEEDING!” (which is a hard line to sell, really).

A number of the frequently-named movies in this thread are at the top of my funniest list (Airplane, South Park, Office Space and Life of Brian certainly), but I’m going to give a shout-out to Rat Race, which certainly had me rolling in the aisles. No one has mentioned it yet, and I feel it is easily as worthy as many films mentioned here.

The Big Lebowski, of course.

A Fish Called Wanda. . . maybe second.

Good call. Not end-to-end Funniest Ever (for me), but the squirrel scene with Kathy Bates was one of the funniest I’ve ever seen.

For me, it was the cow hitting the windshield of the bus that put it over the top.All that setup, and the perfect payoff.

I just wish they had been able to get Boris Karloff to recreate his stage role in the movie (not that Raymond Massey didn’t do a good job, he did.) I am very glad that Bob Hope passed on the lead, and it went to Cary Grant. Otherwise it would have been just another Hope comedy.

I’m contractually obligated to respond to this post, so here you go…

The movie that had me laughing the hardest all the way through was There’s Something About Mary. I almost had to leave the theater on that one.

But I don’t think another favorite of mine has been mentioned yet, so I’ll say Bad Santa. The scene where Willie punches out the reindeer, and then passes out, and the security guy tells Marcus to take him out to the car makes me laugh so hard I can’t breathe, even now after seeing it a dozen times.