Vertigo: Hitch's masterpiece can come off as cold to some on first viewing. Watch it again. The film is an amazing character study, and the ending is absolutely devastating.
2001: A Space Odyssey. Again, cold at first viewing (to some), but watching it more than once reveals some amazing subtext, irony and meaning. It's also beautiful.
Moulin Rouge! An amazing film, but the modern songs may be jarring, as well as the editing and directing style. Watch it again, and you will see that the film is not only good but incredibly entertaining and underrated.
Citizen Kane. Duh. Watch it once for the directing, watch it again for the dialogue, the again for the performances, and again for the cinematography… It's a film school in a box.