1. 2001: A Space Odyssey- It's slow moving at times, but the interaction between Dave and HAL showing the way we blur the line between man and machine and the transition to that from the completely animalistic behavior seen at the beginning of the movie.
2. One Flew Over the Cuckoo's Nest- I know Kesey HATED the movie for not being portrayed through the eyes of Chief, but it's still an outstanding movie. Jack Nicholson's best performance without a doubt....except maybe in Easy Rider hahahahahaha.
3. Star Wars Trilogy- The best greek epic ever to take place in space...errrr....well, the only one, but it's still fuckin' sweet. Too bad none of those actors made it out alive, beside good 'ol Han Solo and I guess James Earl Jones, but he was only a voice actor.
4. Requiem For a Dream- I love anything Aronofsky has done, if you like his stuff check out Pi, he made that movie on $60,000 he raised from friends and family, best movie about math ever.
5. Slapshot- Paul Newman said it was his favorite movie he'd ever worked on. So many great quotes, though it's much greater appreciated if you've ever spent time in a locker room. Old time hockey? Eddie Shore?
I could put soooooo many more movies here and 4+5 could be replaced with "better" movies, but those two are near and dear to me for oh so different reasons.