Okay, in no particular order:
Maybe an all time fave. Hard to commit in such good company. Dark, unassuming and full of original creepy stuff.
The Matrix. (First movie only)
Contender for all time best. Has it all: Originality, hero action, on the edge SF. A bit too many bullets maybe but....
The Day The Earth Stood Still.
Gort Rules. Michael Rennie as Klaatu nails the cold war alien.
2001:A Space Odyssey & 2010.
While very different, both films create aliens we can't understand and pit man against himself while trying to understand them.
Star Wars (Episodes IV, V, VI)
The Skywalker trilogy was brilliant then and brilliant now.
Love the retake on War of the Worlds with a 'virus' killing the aliens who just don't care who we are.
Another great alien that is beyond our comprehension. Well told tale.
Terminator 2: Judgement Day.
Sure, you need T1 for backstory but this one is the balls for great action SF with cyclical story.
Different kind of SF. Too close to reality. Sharp as a razor.
With the exception of the robot cat, this is a great story of man vs. environment. No monsters, no evil genius, no insane computer. Just wits against the wild.
A couple of honorable mentions:
Planet of the Apes (1968), Sleeper, Close Encounters of the Third Kind, ET: The Extra-Terrestrial, Star Trek IV: The Voyage Home, Bladerunner, The Handmaid's Tale, Galaxy Quest,
Alien, Forbidden Planet.
Stay tuned for top 10 novels...