Moon. Made on a comparative shoestring but you'd never think so watching it. A nice bit of high concept fun and very engaging.
As somebody has already said, Fifth Element is a good 'un- worth it for the wardrobe alone. If I was ever making a sci-fi epic I'd have Jean-Paul Gautier do the costumes too.
If you don't mind cheese you could do worse than look into some of the utterly off-the-wall schlocky sci-fi films that were pumped out by the ton in the 50s. The excitement of early space travel charged the imaginations of writers -good and bad- so hard it practically leaps off the screen. Plan 9 From Outer Space is just deranged. On the opposite end of the quality spectrum, early colour masterpiece The Forbidden Planet is well worth watching.
On the subject of the 60s and grainy colour- how could I forget the original 'War Of The Worlds' Hollywood update? It's like Independence Day but with all the cheese and patriotism replaced by a genuine sense of awe and, yes, fear in the face of a seemingly invincible and truly alien threat
Running ahead to the 70s... V. The miniseries, but not the series that followed. Turns out you can have too much of a good thing.
The 80s have been well covered- Alien, Predator, Blade Runner you've seen. Ditto for the more recent past.
I would recommend Star Trek 1-6 (yes, including 5- it may be crap but it's still entertaining) although, like Star Wars, the Trek films aren't so much sci-fi as they are adventure films that happen to be set in space.
Holy crap! How could I forget Total Recall? And not just for the triple titted hookers and 80s Sharon Stone in fitness gear! It's a quality production even without the eye candy. Actually, any of Verhoeven's sci-fi productions are worth watching. The Thing (if you have a strong stomach and don't live alone) and Robocop (2 isn't bad, but skip any further sequels) spring to mind.
-Nick
EDIT: Good call on Hitchhiker's Guide (the TV version) and Close Encounters, cross-posting buddies!