Genuine Science Fiction? I tried for the ambitious, even if most folks think they’re failures.
2001: A Space Odyssey
Blade Runner
A.I. (Yes, THAT A.I.)
All three were met with either lukewarm receptions or outright hatred when they premiered. 2001 is now regarded as a masterpiece. Blade Runner is also highly regarded. A.I. is still regarded unfavorably (especially for its ending) but I find it fascinating.
Runners-up:
Planet of the Apes (Just the original.)
Alien (This one is kinda marginal where science is considered. But very believable.)
Solaris (I’ve only seen the Steven Soderbergh version)
Moon (Unfortunately, you can only really watch it once. Maybe twice.)
Silent Running (Bruce Dern plays another nut. But I think the Valley Forge ranks up there with the Discovery for interesting, hard-science ships.
Like an idiot, I completely forgot The Day the Earth Stood Still. I’m torn between the top tier and runners up.
Forbidden Planet was also something of a watershed motion-picture, but that sits right on the edge of “space fantasy.”
The Star Trek pilot episode The Menagerie is also superb, but also skirts space fantasy.