In no particular order beyond as I scribbled them down as they came to mind…
Blade Runner: No, I’m not going to specify Original Theatrical or Final Cut.
Alien: The only movie that ever fucking terrified me as an adult.
Aliens: So damn quotable
The Matrix: Out-paranoids Philip K Dick, and that’s saying something.
The Andromeda Strain: just so well done and all too plausible.
Forbidden Planet: The best Star Trek film. Monsters…monsters from the Id!
The Empire Strikes Back: Adds that layer of Joseph Campbell mythic element that gave the series heroic depth.
The Thing (Carpenter): What the fuck is that? Too graphic for it’s day.
The Day the Earth Stood Still (not the remake): 50’s red scare style paranoia meets parable with a message.
2001: A Space Odyssey: I think modern audiences will find this too measured, too slow-paced, but it’s effects still hold up today, and well…it was so cosmic!
Robocop: A simply action movie that had some surprising poignancy. Murphy’s death and the salvaging of at least some measure of his lost humanity in a world all too often inhuman raised it above it’s genre.
Dark City: Let me specify The Director’s Cut, which removes the awful voice-over that beats you over the head with what should be an unfolding mystery (and is, in the director’s cut).
A Clockwork Orange: Uncomfortably disturbing.
Mysterious Island: Harryhausen! Bernard Hermann! Captain Nemo! Damn!
The Road Warrior (aka Mad Max II): stripped down to the essentials, I like that it’s characters are defined with such economy.
Wait…I think that’s 15. Some other thoughts and honorable mentions:
Wall-E doesn’t quite make it, but the haunting loneliness of the opening is simply one of the best set-pieces in film.
Brazil: I love that industrious, bustling workplace that suddenly goes quiet and still the moment the foreman closes his door. It’s that bent Gilliam dystopia.
Someone mentioned a favorite from my youth, Robinson Crusoe on Mars. I don’t think it holds up, but I did enjoy it growing up.
Terminator. Maybe if it was the top 20.
Star Trek II: The best of the Treks (well, not counting it’s inspiration, Forbidden Planet). The story reinforces it’s thematic elements with skill and depth. They just kind of ruined the effect with the next Trek. Another tough one to leave off.
Invasion of the Body Snatchers: Okay, so I couldn’t make up my mind and didn’t want to take up two spaces with it. See either of them, they are both outstanding. Also, I wasn’t exactly sure if this was more horror than science fiction.
Minority Report: pity that Spielberg had to put those goofy comedy relief elements into it, or I would have placed it in my top 15. I still enjoyed it, though.
The 12 Monkeys: Well done, but at the same time, I just plain hate the premise that you must helplessly go to a preordained fate.
Metropolis: Hard to watch silent movies at times, but this had some rather interesting elements for it’s day.
Frankenstein: It’s alive! Though I mention it last, it’s only because I cut it out as the very last change, so I suppose it’s 16th on my list.
Edit in: Oh, Silent Running. No, not in my top 15, but certainly an honorable mention.
And further edit in: The Time Machine. I liked the changes they made to the book so it became more than simply Wells speculations and warnings of the future and more about the characters. I like some of the elements of the remake and it grows on me more than when I first watched it, but still prefer the George Pal original.
Oh, and double-damn, I’m the only one with a Harryhausen movie? Maybe you had to see it in the theaters when you were young and it was first released…