John Carpenter's 1982 masterpiece The Thing. The themes of paranoia and isolation are so perfectly explored; it launched the career of Keith David, who is just a treasure; the performances are all immaculate; and those effects. My god, the effects.
That's a great one as well. That alleyway fight scene is so fucking cool. Carpenter is easily one of the most creative, most fun artists of his generation.
I think it's intentionally ambiguous. For me, the point is the paranoia and distrust. I might be wrong, of course, but my interpretation is that we are supposed to leave the experience with questions.