What are your underrated science fiction movies?

Dark City - the Matrix before the Matrix came out, by the director of the Crow, and one helluva lot scarier.

"Mr hand?" "No more Mr. Hand."
- "And Mr. Quick?"
  "No more Mr. Quick. Mr. Quick dead, yes... Poor poor Mr. Quick"

13th Floor and Equilibrium are both also very good.

13th Floor came out about the same time as The Matrix and had a similar feel, but was still very good.

Equilibrium had some of the best gun fighting sequences I’ve ever seen.

Two of my favorite movies - and I'd add eXistenZ to the list, along with 13th Floor and Matrix.
I heard Equilibrium was stifled by people who opposed the anti-religious nature of the movie. The more I think about it the more it makes sense. It could have been a much bigger deal at the time. (Same goes for The Golden Compass.)

Innerspace
Weird Science
Tremors

Would flight of the navigator fit in there?
- Most definitely. So would Super 8.
Innerspace
I LOVED this movie when I was a kid. I need to find a copy and give it a watch as I haven't thought about it in years.

The Day the Earth Stood Still - the original.

Also, the made for TV movie The Langoliers.

The Langoliers needed to be trimmed down. It was broadcast as two large chunks: it needed to be leaner. Still, Bronson Pinchot uncontrollably tearing strips of paper will always be a haunting image.
The Day the Earth Stood Still (1951) is amazing movie.
It's more fun from Klaatu's perspective.

Love Repoman and They Live! Culty classics. Moon, Sunshine, Dog Soldiers (though its more horror action), Paul (comedy sci-fi), attack the block, Threads... Best genre for me.

Repoman the generic opera is amazing!!!! I haven't thought about that movie in a while.
- "Repo Man" (1984) and "Repo! The Genetic Opera" (2008) are two very different movies. Both highly recommend though.
- I'm sure OP is talking about the 1984 movie Repo Man
  https://www.imdb.com/title/tt0087995/

"underrated" and "movies that never get the credit they deserve" seems a bit hard to define. But at least subjectively I disagree with placing some of these movies on this list.

Asteroid City: can a movie that has just barely come out even be considered "underrated"? Feels like not enough time has passed for public opinion to truly form. Besides that it feels like it gets plenty of credit being a Wes Anderson movie.
Under the skin: bombed in the box office as far as I know, but imo also isn't easily accessible for the average viewer and not mainstream. But it does seem to get plenty of praise from the critic side, so I don't see it as underrated from this perspective.
Solaris and Stalker: again I don't think they are underrated by critics and are highly praised. Just not movies that are easily accessible for the average viewer.
Paprika and ghost under the shell: are both a bit niche simply by being anime, but at least from the critic side I don't see them as being underrated. I think ghost in the shell is reasonably known, Paprika maybe a bit less. So that could fit.
Donnie Darko: is a cult classic that definitely has a decent sized following. It's maybe not a mainstream hit, but imo I wouldnt call it underrated.

What I would agree with is "Ad Astra". Definitely flawed and not perfect, but I still liked it quite a bit. So at least from my biased perspective it would count as being underrated

Asteroid City's "play within a play" and that unnerving chant at the end was it's undoing. None of that needed to be there, but the movie would have been too short without it.

I really enjoy classic scifi.

The American Astronaut (my favorite movie of all time)

Until The End Of The World (1991 - director's cut - my second favorite movie of all time)

The Time Machine (1960)

The Andromeda Strain (1971)

This Island Earth (1955)

When Worlds Collide (1951)

Lifeforce (1985)

Pretty much all of the ones referenced in the opening theme for Rocky Horror.. Day of the Triffids, Tarantula, The Invisible Man, etc..

Code 46, Dagon, Interstate 60 (kinda),

Recent gems have been Shin Godzilla, Aniara, Vesper, Doors.. I am positive I am forgetting some others..

Love The Andromeda Strain (1971).

But also:

Pitch Black
The Man from Earth
Timecrimes
Europa Report

Time After Time, in which H.G. Wells invented a time machine that was stolen by Jack the Ripper to escape to San Francisco, 1979.

That one had a recent Bluray release that looks fantastic. And a great movie, with the recently passed David Warner in classic form.

demolition man is exactly correctly rated by me.
but imdb underrates it .

The Day the Earth Stood Still - the original

Saving you a click, here's the list:

    
Asteroid City (2023)
The Andromeda Strain (1971)
The Prestige (2006)
Paprika (2006)
Under the Skin (2013)
Stalker (1979)
Westworld (1973)
Dark City (1998)
Ad Astra (2019)
Annihilation (2018)
Attack the Block (2011)
Solaris (1972)
Life (2017)
Silent Running (1972)
Soylent Green (1973)
The Mitchells vs. The Machines (2021)
The Iron Giant (1999)
Repo Man (1984)
La Jetée (1962)
They Live (1988)
Them! (1954)
The Thing From Another World (1951)
Ghost in the Shell (1995)
Donnie Darko (2001)
Nausicaa of the Valley of the Wind (1984)

With the exception of Them! or Life, I'd argue that none of these are particularly "underrated". To reach that category, I'd say that a movie would need to have a less-than 60 rating on Rotten Tomatoes or other similar review aggregation site to demonstrate some degree of negative or "meh" critical consensus.

Case in point: Enemy Mine (1985).

Ah, yes. Ghost in the Shell, Them!, Solaris and Dark City are truly the underdogs in movie history.
Enemy Mine is totally underrated. the "Best #x movies of Sci-fi" always turns its back on Enemy Mine. That movie has heart.

I loved The Last Mimzy, kids find some "toys" that give them powers. It's cheesy but damn if its not wholesome as hell.

Let's focus on movies that are about the end of humanity.

Virus: The End

Produced by Japan's Toho (famously the studio of the Godzilla films), it's an end-of-the-world flick featuring a frankly astonishing international cast in what could be considered a "conference room drama" -- bottled-up high stakes human interaction in a true dystopic end to humanity. I can't figure how I never saw this back in the 80s; I only discovered it recently and I was blown away.

I'll second The Andromeda Strain, it's also a "conference room drama" and it really works. James Olson was a hugely underrated actor of the era.

Colossus: The Forbin Project, also wire-taut conference room drama with imminent destruction hanging on every decision.

And while we're at it, War Games, probably the best known of all these films, and maybe the only one that doesn't merit the categorization as a conference-room drama. But the stakes are the same.

Titan AE (2000) it bombed hard at the box office but damn is it a beautiful, fun and interesting movie.

All hail Planet Bob!

My underrated picks would be Melancholia (2011) about a planet passing unsettlingly close to the Earth during a wedding, and Coherence (2013) about a group of friends gathering for a dinner party on an evening when a comet is passing overhead.

Typing this out I realise they actually both sound quite similar but I promise you they are very, very different and worth a watch.