Favourite SF movie of all time would have to be ALIENS. The tough as nails marines, the fact that Ripley finds her maternal side in unexpected places, the hardware, and that final battle - "GET AWAY FROM HER, YOU BITCH!" I absolutely love it.
I watched it the other day, and it's hasn't dated badly at all.
THE FOUNTAIN is another favourite of mine that no one seems to have heard of. Beatiful themes, tragic story, and amazing visuals of a truly transcendent spaceship flying towards a distant nebula.... unforgettable...