re: What's the funniest movie

If I had to pick just one I would say Monty Python and the Holy Grail but there are probably fifteen to twenty that are all timers.

Holy Grail
A Shot in the Dark (deeply disappointed no one has mentioned this one, the best of the Pink Panther films)
It's a Mad, Mad, Mad, Mad World
The Life of Brian
Airplane
This is Spinal Tap
Blazing Saddles
Young Frankenstein
A Fish Called Wanda (only movie I know where someone watching it actually laughed themselves to death)
Dirty Rotten Scoundrels
The Naked Gun
Caddyshack
National Lampoon's Christmas Vacation
Dr. Strangelove
The Blues Brothers