The first films that come to my mind when i think in film history (and there are piece of art in general) are:
Citizen Kane by Welles, because established the rules of modern cinema
Rebecca by Hitchcock because is like an art movie class by itself
and at last i could think in a genious like Stanley Kubrick if couldn't be he was just an apprentice of Max Ophüls, pioneer of scenes with traveling and plane sequences. Any title of his filmography is a masterpiece.