Lots of great picks so far; here's 10 of my own:
Clerks (among the top 10 best black and white films I've ever seen, like Arsenic and Old Lace or Young Frankenstien funny).
The Exorcist (scariest movie I've ever seen to this day, and I caught it opening weekend).
The Godfather
The Godfather II (which was better? don't know and don't care; both were fantastic and needed each other equally to be whole)
The Unforgiven (if Clint ever made movies for a living, he made that movie to live forever)
Jaws (it freaked people out like an Orson Wells radio show, no shit)
Patton (the great war movie among greats like The Great Escape, Saving Private Ryan, Stalag 17 etc.)
Star Wars (it's so campy and yet so great that my grandchildren's kids will still follow that franchise long after I'm dust)
Apollo 13 (only because it actually happened and the acting was fantastic)
Cinderfella (the story Hollywood tells more often than any other, as only Jerry Lewis could do it)