Sigh... this is such an unfair question... here goes anyway:
American Beauty (it's a really comforting movie, actually, with so many "essential truths"
Airplane (was my favorite until I saw American Beauty)
but this question is not fair because there are SO many movies that are just a micrometer below these... I like a lot of the movies that have already been mentioned in this thread. I would also like to add Brazil and Rushmore.
Oh, and a little movie called "Harold & Maude"... it's not for everyone, but if you like it, there's a good chance that we'd get along well.
l2c