"The Shining" and another called "The Cube".
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.