Forbidden Planet - I have watched this as a kid, still want to see it again. The plot was amazing.
War of The Worlds - Watch Both versions, original and Spielbergs.
2001 - The pinnacle of hard sci fi films.
Solaris - Watch it in day you are in a good mood, because you will need that, but worth of course.
Clockwork Orange - A distopian proto-cyberpunk classic.
Westworld - get some wild androids going nuts.
The Andromeda Strain- Really astonishing film for its time.
Alien 1- The best one ever made about killer aliens
Outland - a western in space
Blade Runner - Best film ever (IMHO of course :-))
Predator 1- though not strictly sci-fi, the monster is an alien right?
Akira- Cyberpunk anime. A classic.
Steamboy- Ottomo rocks again!
Total Recall - P. K Dicks based film with roller-coaster action and Sharon Stone.
Sphere- Highly underrated but really impressive.
Contact- A very good version of Carl Sagan's excellent book.
Dark City - One of the most underrated films ever. Must see it.
13th Floor - part of the not connected "fake worlds" trilogy with the above and below mentioned films.
Matrix 1, 2 and 3 - You probably watched it already.
Ghost in The Shell- Maybe the best cyberpunk anime ever made.
Fifth Element - The best space opera ever made. Astonishing visual.
Gattaca - Genetic engineering themes.
Serenity - Watch the series first though: Firefly
Mad Max 3- Incredible settings and landscapes in the desert.
Minority Report- Check the eye scene.
AI: Yeah I pretty much dig it.
The Chronicles of Riddick- This is a futuristic fantasy but hell worth it.
Wall.e - Really astonishing film though people think it is aimed to kids only.
Sherlock Holmes 1 and 2- Pure steampunk detective story.
Inception- Journey within the mind and dream.
The Prestige- Incredible steampunk, magicians and Nicola Tesla!
Watchmen- Not as good as the Comics, but far close.
Signs- Not strictly a film about Aliens, but about redemption and faith.
The Incredibles- Well, a superhero film with huge dieselpunk tones.
The Iron Giant- A great dieselpunk film for the family, but very smart.
Star Wars 1 to 6- Well, should I justify why ?
Back To the Future 1 to 3- Well....
12 Monkeys- The best Terry Gilliam film by far. Distopian realities as always.
John Carter Of Mars- Andrew Staton excellent space and swords film!
Watchmen- I cannot dislike it even face to the comics.
V For Vendetta- The very same thing as the above
Prometheus- It was excellent!
Total Recall 2012- As good as the 1990 one!
Looper- Quite surprising film with great time travel concepts!
Cloud Atlas- The Wachowskis are back !
These are my favorite ones, I am not even calling them "best", because it would be frakkin arrogant ;-)