I'd have to go with 2001: A Space Odyssey. I saw it when I was 17 and had heard a range of reactions from the best movie ever to the most boring movie ever. I've seen it so many times, and despite its slow pace it never gets dull. Recently my friends and I got baked and watched the third part in a dark room while Echoes played synched up on a cd player. The only other time I've felt that entranced in a film was watching Anemic Cinema, but that wasn't near the extent of Space Odyssey.
http://www.youtube.com/watch?v=anjVjj4w5rA is one of my favourite scenes in any movie.
There are other movies that I don't get tired of watching like Eternal Sunshine, Block Party, Dr Strangelove, Breathless, or Pulp Fiction, but none of them are as good as 2001.