HomerSimpson: I would endorse Carpenter's The Thing, as well. Also, if you haven't seen it (and I'd be surprised if you haven't), you might want to check out the original
Alien. One of the best sci-fi/horror films of all time imo.
GoJays2025: Agreed on Alien. One of the few movies to terrify me beyond belief, although watching it at the age of 7 or 8 probably contributed to that.
Another movie that scared the crap out of me was Event Horizon. When I was about 14 or so I was home alone one night and saw it come on TV. Thinking it was like a harmless sci-fi movie, I stayed to watch... afterwards I literally could not sleep for 3 days in a row. Thinking about it still gives me the creeps to this day.