Unless I’m mixing it up with something else, that has to be the most egregiously disgusting, stupid, stunningly wit-free film I’ve ever seen in my life. Doesn’t it feature (what seemed to me) to be ridiculously lengthy scenes involving defecation that I presume were supposed to be perceived as hilarious?
One … Two … Three - the funniest of Billy Wilder’s movies, amazing timing, perfect use of Cagney, and best “spy exchange” ev-AH! “The situation is hopeless, but not serious.”
Now there’s a movie with product placement!
Rat Race.
Rowan Atkinson: “I am een a rrrace…!”
Another for A Funny Thing Happened On The Way To The Forum.
La Cage Aux Folles. (The original 1978 film.) I laughed so hard my glasses misted up.
Planes, Trains, and Automobiles
If I wanted a joke, I’d follow you into the john and watch you take a leak.
BRINGING UP BABY with Cary Grant and Katherine Hepburn
I love a lot of these, and Ghostbusters and A Fish Called Wanda are up there, but I think I need to throw a new name into the running, a movie I can watch over and over and laugh out loud:
Noises Off is my choice. Amazing cast, an overall tone of zaniness, lots of physical comedy, but also moments where, for instance, Michael Caine just saying “No,” in the perfect tone of voice gets me rolling.
It’s A Gift
I watched this on the basis that it was written by Francis Veber, the writer and director of my favourite comedy Le Dîner de Cons (which I previously mentioned here). Despite having read many good things about it, I couldn’t believe what an insipid and unfunny snoozefest it was. I don’t think any of the people I was with even bothered to see it through to the end.
What I didn’t add was that I saw it again recently, and, yes, it was totally unfunny. But I haven’t let that detract from the endless laughter I got from it first time around.
I’ve found that there are two potential problems with comedy: one is that, in common with many other things, it goes in fashion cycles, and, second, a lot of it doesn’t bear repeated viewings - or hearings.
This was the film I’ve laughed the hardest at. I saw This Is Spinal Tap in the theater, at a screening sponsored by KY-102. The thing is, this was before the film had received any publicity, and a large part of the audience didn’t get the joke. They thought they were watching a documentary about a band they thought they were vaguely familiar with.
I was laughing so hard that I had to leave my seat and go to the lobby to resume breathing right after the bit about the fate of the various drummers.
I don’t know if it is the “funniest”, but at that time, in that circumstance, I laughed hardest.
Jackass! - not sure it qualifies, but I’ve never laughed that hard during any other movie.
Philadelphia Story was in my top three too. Post + handle = confirmation that I chose correctly. 
The Jerk
Vacation
I am not old or white.
Ooo, that’s tough. If I could only pick one, it would be:
Trading Places
Eddie and Dan and Jamie Lee and Denholm and “the brothers.” Wow! Great cast, great lines, a great scheme, and a wonderful scene of sweet, sweet revenge at the end. I’m surprised it hasn’t been nominated yet.
Other comedy favorities of mine include:
*True Lies
The Big Lebowski
Monty Python and the Holy Grail
The Life of Brian
Paper Moon
Blazing Saddles
Airplane!
GalaxyQuest
Raising Arizona
Ruthless People
What’s Up Doc?
Dr. Strangelove
Spinal Tap
Groundhog Day
Dumb and Dumber
Animal House
Dirty Rotten Scoundrels
Ghostbusters
A Fish Called Wanda
Bruce Almighty
Liar, Liar
Time Bandits
La Cage Aux Folles *(either version, actually)
The Ladykillers (ditto)
The sadly underrated: