I've only ever seen 3 movies that I actually considered scary, and Poltergeist doesn't really hold up when you watch it again now.
Play Misty For Me is my sleeper pick for HOLY FUCK TERRIFYING, possibly (probably) because of a lifetime of ignoring Rule #3.
But the scariest movie of all time? Fuck you if you don't think Aliens is some paralyzingly frightening shit. I saw that movie in the theater when it came out (I was 9 and my mom thought I was seeing some dumb kid movie) and I swear I didn't sleep with the lights off for a week. And it's the only movie that's possibly gotten even scarier as I get older because there are so many LAYERS of horror there and the best ones aren't even comprehensible to a little kid.
Wait, Alien or Aliens? I fully agree that the first was a great horror movie. The second was always more of a sci-fi action flick to me. Not scary really, but great nonetheless.
For frightening space movies, I'll always have a soft spot for Event Horizon. Something about watching that just really creeped me out.
Alien was OK, but hardly scary.
Aliens is what I was talking about. Flat-out terrifying, in addition to being one of the 4 or 5 best movies ever made.
Event Horizon was terrifying when I saw it in the theater on several hits of LSD, but I found it didn't really hold up to rewatching.
I, too, prefer political docudrama Aliens over its lighthearted feminist comedy predecessor Alien, since we're just being silly now