Godfather 1 and 2 are probably my favourite films.
I would struggle to just come up with another three but, off the top of my head, Clockwork Orange, American History X and Apocalypse Now are great.
Hounorable mentions to Green Mile, Forest Gump, Shawshank Redemption, Full Metal Jacket , Gladiator and heaps of other great films that I can't think of right now.