My favorite sci-fi films.

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 pinacle of hard sci fi films.
Solaris - Watch it in day you are in agood 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.
Total Recall - P. K Dicks based film with roaler-coaster action and Sharon Stone.
Sphere- Highly underated but really impressive.
Contact- A very good version of Carl Sagan's excellent book.
Dark City - One of the most underated films ever. Must see it.
13th Floor - part of the fake worlds not connected trilogy with the above and below mentioned films.
Matrix 1 - You probably watche 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
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.

These are my favorite ones, I am not even calling them "bests", because it would be frakkin arrogant ;-)