Blade Runner is a fantastic movie, but I think that the greatest sci-fi movie thus far would be Star Wars, the original (A New Hope), from 1978, the un-enhanced version. That movie took off for a reason... it was a classic story, that people around the world could relate to and embrace.
It's hard to believe that George Lucas went on to create so much drivel. Empire was awesome, Jedi was fair, and let's not talk about the "prequels"... At the beginning, when he had something to prove, he was at his best.
@Stimpy - I almost picked Alien...