Space balls
Blade Runner
Strange Days
Total Recall
Aliens
Logans Run
Star Wars
Predator
Space Odyssey
Clockwork Orange (not sure if it qualifies as a Sci Fi)
and of course the best of all time...Plan 9 from Outer Space. Bela Lugosi actually died during the filming of this absolutely horrid flick. Naturally he was playing the Dracula like character.
Brother from Another Planet wasn't really a Sci-Fi movie, but was very funny.
The Man Who Fell to Earth is not really Sci-Fi, but was interesting, if you like Nicholas Roeg. (Anybody remember another Roeg movie, Performance, with James Fox and Mick Jagger? Great movie!)