Gattaca, no doubt
Trying to pick things not already mentioned . . .
Ghost in the Shell and Starship Troopers are both great but for very different reasons.
I also really enjoyed both The Postman and Waterworld, despite all the criticism they get, though possibly not the kind of science fiction you're looking for.
For something modern with an atmosphere you could cut with a knife, Upside Down.
+1 to many of the suggestions here, but also Primer is fantastic if you can track it down. I don't think I saw 12 Monkeys listed yet.
awesome, thanks for this thread! Seems like a few decent sci fi films out there I haven't even heard of yet. can't wait to go home and be a pirate yohohoooo
Pitch Black
Alien and Aliens(3 and 4 were garbage, skip em)
Predator series(the original is a classic, Predator 2 is really dumb but still kinda fun, Predators is a step up from 2 but still not as good as the first, skip the Aliens vs Predator crap as they suck)
Pitch Black
The Thing(1982 version, though the original is also good IF you like classic 50's sci-fi)
Event Horizon(glad to see this one getting some love here)
Terminator 1-2
Robocop(1st one only)
The Abyss
Cube
Tremors(sequels aren't nearly as good as the first)
The Matrix
Inception
Blade Runner
Equilibrium
Dark City(really deserves to be seen by more people!)
Serenity(helps if you've already seen the Firefly series)
District 9
Total Recall(Arnold version, haven't seen the new one)
12 Monkeys
I, Robot
They Live(action, sci-fi, and satire, meant for the Reagan-era but still applies today)
Escape From New York
Outland(it's basically High Noon in space, with Sean Connery)
Trancers 1-2(fun, low budget, sci-fi that's actually kinda unique)
These films are "old" but I wouldn't call them "shite" films:
Godzilla/Gojira(the original was a much darker film than the ones that came after, and should be seen, preferably the subtitled Japanese version. The 1985-1995 Godzilla movies also had a darker tone, overall, especially Godzilla vs Destroyah, which is rather grim)
The Beast From 20000 Fathoms(this was one of the movies that really inspired Godzilla)
Them!(sure, the effects are way behind modern levels, but the movie is remarkably suspenseful for it's time period, and has one of the most memorable sound effects in history)