Most of the funniest I can think of are listed already.
So I'll just tell a little story, of the funniest movie I can say I saw in the cinema. Borat.
I went to see it, later session on a Sunday evening. I was there by myself, there was one other couple in the whole place. After the film, I couldn't stop laughing - tears of laughter, the works. It's a big multiplex cinema place where you walk through the foyer to get out. So I had to go into a bathroom area, it still took me several minutes to compose myself, I'd almost have it together, then I'd think of some other part of the film and loose it again. I was, after a while, finally able to get a straight 'poker face' and walk out.