It is extremely difficult to pick one movie since you usually cannot compare them. What I like in comedies differs e.g. so much from what I like in action movies or dramas. But if I had to pick one it would be Harold and Maude http://www.imdb.com/title/tt0067185/ This movie has almost everything: a beautiful love story, depth, black humor, funny ideas, good soundtrack, surprising ending and even a car chase.
My top 5: 1 Once upon a time in the west 2 Back to the future trilogy 3 Planet terror 4 Akira 5 fight club (6 die hrad trilogy 7 sin city etc etc :P) None can accuse me of being tedious :P
And a special price for 2001, The Usual suspects, Pulp fiction, Barry Lyndon, Happiness, Buffet froid, The Thin red line, Barton Fink, Préparez vos mouchoirs, Once upon a time in the west and Reservoir dogs.