While I'm liked almost all the ones already mentioned I was holding off on chiming in to see if anyone would bring it up. Top notch suspense/horror movie. Could have been a Philip K. Dick story.
Others I haven't yet seen mentioned (haven't read everything yet):
For pure fun horror "Return of the Night of the Living Dead" and "Night of the Comet".
Doubt if it's held up but at the time Lifeforce struck me as a pretty good sci/fi horror flick. Not in the Alien category, but hey, I'll watch just about any scary movie.
I just like the "lady space vampire!"
I've liked Steve Railsback too, especially in "The Stuntman," although that's not horror.
And "Captain Picard" really gets his in that movie (Lifeforce) too! lol
Mark