A semi-outdated list:
2001: A Space Odyssey, A Clockwork Orange, Apocalypse Now, Brazil, Citizen Kane, City Lights, Cool Hand Luke, Duck Soup, It's A Wonderful Life, Magnolia, Metropolis, Modern Times, Mr. Smith Goes to Washington, Snatch, Soylent Green, Star Wars: The Empire Strikes Back, The Good the Bad and the Ugly, The Great Escape, The Machinist, The Third Man, The Usual Suspects, THX 1138