When I think of this category, some of the ones that immediately spring to mind are:
Gattaca - A modern classic destined to only become more and more topical as time goes on, visually lovely, well acted, smart ideas, and overall one of my favorites. This is the first one I think of what I think of "smart sci-fi."
Children of Men - People fawn all over the two long takes in the movie for good reason - they're fucking amazing - but the movie is equally awesome for its writing and themes. The hook is pure sci-fi (humans can't have kids anymore), but what the movie is REALLY about is immigration, xenophobia, and what happens when a society teetering on the edge of collapse is asked to have empathy for others.
2001: A Space Odyssey - I mean, duh.
2010: The Year We Make Contact - Yeah, it's got "1980s" written all over it, and it suffers in esteem due to being set alongside 2001, but this is a great movie all the same, and by god do those special effects hold up.
Arrival - A recent one, this movie initially appears to be about first contact, then appears to be about communication and language, but in truth it's about the choices we make and the extent to which suffering is a desirable part of the human condition. This film made me cry. It's a terrific film that I highly,
highly recommend (just be sure to go into it blind).
Code 46 - An overlooked and underrated little gem that covers similar themes as Gattaca, it's a rare Tim Robbins movie I like, directed by Michael Winterbottom.
Blade Runner 2049 - Very recent, the fact that this was a box office failure pained me greatly, because this movie was fantastic.
I loved it. In fact, I'll go ahead and say it: I liked it more than the first. There is a LOT to unpack here. This is one of those movies that leave you silent for an hour after seeing it, then you just want to unpeel it all with someone, which is exactly what smart sci-fi should do.
12 Monkeys - A twisted modern classic, this is actually my favorite Terry Gilliam flick. It's strange and bizarre and is as much fantasy as it is sci-fi, and I'm not sure it holds up thematically compared to others on this list, but I'll be damned if it isn't engrossing each and every time.
Contact - Already discussed elsewhere
Solaris - Either one, really. Both are heady and weird and force your brain into contortions.
Annihilation - From earlier in this year,
this movie left me scarred, in a good way. It's a shame the marketing was so bad, because this was a fantastic piece of work with a chilling, nearly silent sequence near the end that left me lying awake all night.
Ex Machina - Fantastic, small, focused, contained SF film that deals with the usual questions about what makes us human, whether or not machines can be sentient and whether they should have rights, and so on. Basically, a guy has AI robot servants who appear human, and how he treats them becomes the crux of the story. Love love love this movie. Oh, and it's directed by the guy who did Annihilation. (He also wrote the screenplays for 28 Days Later and Sunshine.)
Some others:
Sunshine - I wanted to like this more than I did. The first 3/4 are a great character study with a shaky science hook, but the final act suffers from the same problem 28 Days Later did in that it feels like a totally different movie invaded this one. And yes, they are directed by the same guy.
Moon - basically, take the HAL 9000 segments of 2001 and make an entire movie out of them. Sam Rockwell is really good in this, and the turns it takes pose some really compelling questions about humanity, morality, free will, and so on.
The Abyss - James Cameron's best work
Eternal Sunshine of the Spotless Mind - Barely sci-fi, perhaps, but good stuff with two great performances.
Enemy Mine - I haven't seen this in years, so I don't know how it holds up, but I remember this being really thought-provoking. Basically, a SF treatise about race and bigotry disguised as an alien movie.
Looper - By Rian Johnson, the guy who made the best Star Wars movie since Empire Strikes Back. Time travel movies are hard to do well, but DAMN does he do it well here. Smart, stylish, gripping, I feel like this movie is really overlooked. It's destined to be a cult classic.
The Adjustment Bureau - Not great, but this is an overlooked Philip K Dick adaptation starring Matt Damon and Emily Blunt that is a pretty enjoyable SF thriller with some decent themes. Not an all-time great, but worth watching and one a lot of people have skipped.
Forbidden Planet - An all-time classic! One of the greats of all time, anyone who likes classic SF who hasn't seen this MUST SEE IT NOW. It's the stuff of legend.
(The first 1/2 of)
The Island - I swear to god, the first half of this Michael fucking Bay movie is pretty great and, yeah, it's smart SF ... that is, until it switches gears and turns into a Michael Bay movie. 100% worth watching, though.
Silent Running - It's tooootally '70s, and the cheese factor is a little high here and there, but overall it's a pretty smart flick about societal responsibility, conservation, and more. The robot effects are bad, but the other stuff is great, done by the guy who did the FX in 2001.
Star Trek: The Motion Picture - Yeah, seriously. Given any other name but "Star Trek" and people would praise this as a cerebral SF film, but since it's Star Trek it gets slammed as being a bore. I love it, though, and place it second only to Wrath of Khan (which is great, but not necessarily "smart.")
Europa Report - A small budget flick about a mission to Europa and what it means for humanity, this is an overlooked little gem that I highly recommend. Not an all-time great or anything, but if you're a SF fan it's well worth watching and one that will make you wonder why it never gets brought up.