I think I would have to pick some more serious movies to be the best of all time. There are a lot of comedies that I like, but most comedies aren't "great" movies IMO; they just provide us with some relief...
"Schindler's List" comes to mind
"City of God" was a pretty cool movie. Foreign film about a kid who grows up to be a big member of the mob and is the most powerful drug dealer (and murderer) in brazil. It's something refreshingly different from most American films, where the ending is often predictable.