This is a tough Call, however I am going to have to say The Outlaw: Josey Wales is the single greatest movie ever produced. There are many other movies that come close, but nothing is quite as good. I have seen this movie more than 20 times throughout my life, and I never get bored of it. The character development is awesome. The story is great. The acting is good. There are lots of good lines, and its full of vengeance driven violence.
I really also like The Shawshank Redemption, Requiem for a Dream, Braveheart, Schindlers List, The Indiana Jones trillogy, The Good the Bad and The ugly, Apocalypse now, Army of Darkness, Full Metal Jacket, Office Space, Young Frankenstein, Blazing Saddles, American History X, Fight Club, Platoon, The Magnificent 7, The Matrix, The Terminator, and Terminator 2 a LOT as they are all among the top of their genre IMO.