Wow, movies... now that is really tough. There are so many I have seen more than once and would see again. Going waaaaaaaay back I would pick Out of Africa and Reds. There are a lot of good scripts, but combined with great cinematography, there are some movies we never forget.
For the timing in motion picture history and its sheer originality I would pick Star Wars, 1977, I think.
I'm always amazed by the wide variety of favorites amongst movie fans. I started watching Blood Diamond and the violence, though undoubtedly authentic, upset me so much I couldn't stick with it. I guess I internalize too much.
I will have to think about this question a bit more. So many movies are worth a mention.