Too many good movies to even remember them all. Lots posted already are fantastic.
Some of the ones posted.. hurt my head. Dumb and Dumber? It was AMAZING! When I was 12. As an adult it's just awful. The most unintelligent forced humor ever. Any movie that ever has an over the top bathroom scene, stopped being funny a long time ago.
Shawshank Redemption might be my favorite movie of all time. I can not find one single flaw with it.
American History X is up there.
Forest Gump, A Beautiful Mind, Gladiator, Braveheart, Saving Private Ryan.. all those rank up there.
Return of the Jedi is to date probably the movie I've seen more than any other. The new Trilogy pretty much killed Star Wars for me though. The original Trilogy is up there.
Ofcourse the Godfather Trilogy is there as well.
There are many others, including a lot of older movies that I can't think of.