It occurs to me that the IMDb poll mentioned below is missing a couple of obvious candidates: A Beautiful Mind, maybe, and Vanilla Sky, certainly.

Obviously, I have no sympathy for anyone who blows the ending to any movie, even a bad one. (This scrupulousness on my part has prevented me from making all kinds of bad Fight Club jokes, for instance.) However, of all the movie endings listed below, I think I'd be most furious at anyone who intentionally blew the ending of Citizen Kane for someone who wasn't already aware of it. It's a miracle to go very far in life without learning the secret of Citizen Kane, and seeing the movie with that surprise intact is a gift that few people are afforded these days.