Hmmm, there are a lot of movies I love that are not strictly comedies, but contain very comedic elements, ie. The Big Lebowski, Pulp Fiction, Kiss Kiss Bang Bang, In Bruges or the Princess Bride. But in the interests of keeping this list as pure comedy as possible, and narrowing down my choices, I'd have to say:
Dr Strangelove Or How I Learned to Stop Worrying and Love the Bomb (A Peter Sellers movie had to make this list, and I considered the original Pink Panther, but I think DSOHILTSWALTB is probably his single greatest film)
A Fish Called Wanda (like Python, but less silly)
Toy Story (I know that's a kids movie, but with that film, Pixar really, really started something big)