Your Favorite Hard Sci-Fi Movies?

I just posted something that brought back some great memories of one of my favoritehard sci-fi movies of all time: Runaway, with Tom Selleck as a bomb squad cop and Gene Simmons (of KISS) as one of the best terrorist villains in cinema hisory. I must have watched that movie a few hundred times when I was a kid. I wish I owned it on DVD. We're just barely one step removed from that today.<br /><br />Other favorites of mine include the obvious: Blade Runner, Alien, Aliens, 2001: A Space Odyssey, Serenity, and I'm sure there's plenty others that aren't coming to mind at the moment.