Gotta agree with many others here:
Top of the list....Forbidden Planet.
Also,
The day the earth stood still (old BW version, the new one is terrible)
Alien and Aliens.
Contact
Blade Runner
Silent Running
Soylent Green
Moon
War of The Worlds (both versions)
The Thing (50's version)
Solaris
K Pax (not quite SciFi, but great)
The Keep
Anything Star Trek!
The Dish
Invaders from Mars (1950's)
The Mysterians (great Japanese scifi movie of '60s)
Outer Limits series
Apollo 13
some that come to mind, but there's lots more
