Stoned = Spinal Tap.
Sober = Monty Python Life of Brian.
So I broke the rules, sue me.
Since Airplane! has been mentioned many times, I’ll go with the lesser-known Living in Oblivion. I rented this with my then-roommate many years ago because it was one of the few films still available. We didn’t know what to expect but ended up having to pause the film several times because we were laughing so hard we were missing parts of it.
Nope, Grizzly Man. Sad ending but it’s a miracle he lasted as long as he did. Sorry, but watching someone get too close to a grizzly in the wild and then say ‘you’re a grumpy bear’ when he bluff charges is just fucking hilarious. Hello Darwin.
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Of all the contenders, I’d have to go all the way back to a silent: Buster Keaton’s The General. I can’t even imagine my second-funniest.
It is 1971, at the Worldcon (the annual World Science Fiction Convention) in Boston. They have a 24-hour science fiction movie room set up. I’m still in college; going to sleep early is not an option. I wander into the room at about 1 am.
For reasons that are obvious only if you understand the minds of science fiction fans, they have somehow snagged a copy of the just released first movie of a British group named Monty Python that only a handful of people in the states have heard of.
And Now for Something Completely Different. It’s hard to laugh when your jaw is scraping the floor from utter amazement, and technically it’s hard to call it a movie. Doesn’t matter. It remains the single funniest strip of celluloid in all history.
The hardest I have ever laughed at a movie was This Is Spinal Tap. I mentioned this in another thread just yesterday, but I was in a preview audience gathered by the local rock station KY-102, who apparently were not aware that it was a satire. Which only made it that much funnier. I was in the front row and had to leave the theater and go out to the lobby to be able to resume breathing, I was laughing so hard I was in danger of passing out. I think I actually fell out of my seat into the aisle.
You didn’t ask, but the hardest I have ever laughed at live theater was Avenue Q.
Airplane. Hands down.
That scene where Elaine is dancing and the guy’s trying to point to the knife in his back and she thinks it’s part of the dance is beyond hysterical. My favorite laugh of the movie.
Lots of great contenders, but going by the rule and picking just one it would have to be " What’s Up, Doc?". This movie has a special place in my heart because I remember watching it new as a kid and while some of the humor went over my head, I recognized and appreciated great actors and a funny story. Plus, seeing it now 40-something years later, it still holds up. Such great actors too, especially Madeline Kahn.
The OP specifically requested ONE movie. Otherwise you’re not really answering the question. Why are people just blowing this off? Why not just start another thread if you’re incapable of choosing just one?
The Jerk
“First I get my name in the phone book and now I’m on your ass. You know, I’ll bet more people see that than the phone book!”
The Adventures of Power.
I’ve never laughed so much at something so absurd.
Animal House would have been my vote for many years until, There’s Something About Mary came out. So many great scenes and lines and the movie still has not gotten old or one bit stale.
I never thought Animal House or some of the great old comedies like Blazing Saddles, Spaceballs, Vacation, Airplane would ever be topped but there are some really great comedies in the last 10 years or so that might surpass that previous generation of comedies.
For an “all time” list, I’ll go by movies that have me repeating lines from them over and over, and for me, there is no other choice:
CaddyShack
+1
I have to admit that the one movie line I most quit is the one I say whenever someone asks me of Madame Pepperwinkle’s plans: “Mongo not know. Mongo only pawn in game of life.”
Last night over dinner I mentioned this thread to the spousal unit and asked him the question. His immediate, uncoached response: The Birdcage (which was mine in the thread).
We make a good pair. 
Sorry, I knew the ending going in. Seeing a guy who was later going to die while being eaten alive and screaming does not a comedy make for me.
Now Project Grizzlyis comedy gold. I snorted at :23 when I first saw it.
Another vote for “The Gods Must Be Crazy”
Airplane! when I first saw it was like comedy nirvana to me. I think upon repeat viewings and age I like Young Frankenstein better, but the peak comedy viewing experience for me was that first viewing of Airplane!.
“Girls Will Be Girls” is the funniest, wrongest, and definatly campiest movie ever made. And it gets
funnier every time you watch it.