John Carpenter's 1982 version of "The Thing."
It's the only movie that freaked me out as a kid. Not because of the visceral practical effects, but because of fear of the unknown, and the extreme isolation of the Antarctic. Knowing that the alien is disguised among you and having no way of escaping it due to harsh environmental factors, to me, represents one of the ultimate forms of psychological torture.
The movie is well paced, Carpenter doesn't throw too many scares at you all at once. They're all planned and executed methodically. Each one having it's own sort of horror "charm." **POSSIBLE SPOILERS** Even at the end of the movie, when you think you have it all figured out, Carpenter leaves it open to interpretation that you still have no idea who is infected even right up to the end (if there is anyone infected at all).
The film's music is an ambient synth collection that reflects the isolated and dreary atmosphere. Since you likely have some decent audio gear, have a listen. ;)
Now, there are other films with similar "enemy disguised among us" plots and there's even another remake of The Thing that came out a couple of years ago, but these movies fail to capture the raw anxiety that the 1982 version did so well at capturing. The characters in these other movies are boring, the scares are cheap, they over do it with the action. Sometimes, especially with horror movies, "less" is "more."
Also, I greatly dislike computer effects. I think they are often over used. What Carpenter did with practical effects, models, and stop motion amazed me. It still amazes me.