“Serial Mom” has to be the funniest movie of all time, in my opinion.
Nothing new, just adding my votes for:
Airplane
The Party
What’s up Tiger Lily
anything Mel Brooks
IMHO “Blazing Saddles” was the funniest movie I have ever seen. A few others that I found extremely funny are: Vacation, The Jerk, What about Bob, Friday, The Stoned Age, and Smokey and the Bandit (the first one, Jackie Gleason had me rolling in the floor)
One movie that gets me going every time I see it is Tommy Boy with Chris Falrey and David Spade. Billy Madison and Happy Gilmore were real good, although I thought The Waterboy and Big Daddy sucked.
I don’t know about the funniest film, but IMHO 
the funniest sequence on celluloid ever made is the football match between the True Blues and the Dirty Yellows in “Bedknobs and Broomsticks”.
It would drive anyone to incontinence!
The rest of the film is Disney cornball stuff, but thankfully videos have fast forward!
So many funny movies…
I’d like to add “Philadelphia Story”. Best scene: Jimmy Stewart standing outside Cary Grant’s house, drunk, shouting: “SDMB Administrator!”
(Sorry)
A more recent movie I absolutely have to mention is “The Big Lebowsky”. The Coen’s best film, at least to me. I fell off my seat during the dream sequences, twice.
And you haven’t seen an audience react like the one at the Tel Aviv theater I first saw it at, after John Goodman shouted “SHOMER SHABBES! SHOMER SHABBES!” Five seconds of stunned silence, followed by mass hysteria. Priceless.