2001: A Space Odyssey
2001: A Space Odyssey is more than just a film, it is considered by many to be an Odyssey of Spatial Dimensions. It is notable for a) Being very good b) Making very little sense and c) Blowing your mind so that you don't have any idea if the pertient questions about humanity, evolution, war and technological advancement are answered or even, in some cases, asked. Some of the problems posed are answered by the sequel 2010, which disappointed many people by onlybeing a very decent science-fiction thriller.
Mark Kermode often says "Stanley Kubrick went from the beginning of civilization to the dawn of a new breed human being in just over two hours". when discussing films that are very long and not very good (e.g. Sex and the City 2 or Pirates Of The Caribbean 3).