It's up there as far as science fiction movies, but for me, nothing can touch The Godfather. Best film I've ever seen.
For science fiction, other good ones are
The Matrix
Space: 2001
Terminator 2
Star Trek 2: The Wrath Of Khan
Blade Runner
Alien
Aliens
Star Wars
Star Wars: The Empire Strikes Back
Avatar
Wall-E
Brazil
The Iron Giant
Iron Man I, II, III
Vanilla Sky
The Abyss
I enjoyed all of those, there's probably more but I'm tired of typing