Well if you mean the funniest screw up, then I'd have to say: Pearl Harbour - I loughed so hard during that movie (just watching the expressions on other people's faces).
Otherwise the best comedies: I go for the really old school!
The Marx Brothers - most of all - The Night at the Opery
non-Marxist movies (
): The Man who came to Dinner, Some like it Hot, George Washington slept here,...
Newer stuff (not really new, just in colour already): Buddy Buddy, High Anxsiety (Mel Brooks rocks!), Also Young Frankenstein (although made in BW), And I laughed really hard at Mr. Been - The Movie, but the ones that really stand out are the Pythons - LIFE OF BRIAN!!! (and the other two of their movies).