The Thing (1982)
Because of EVERYTHING. The acting, the plot, the special effects, the surprises, the tension, the paranoia, the lack of predictableness, the splurge of creativity and originality, EVERYTHING.
I've seen this movie more than every movie, save two. I've shown it to countless of friends, all of whom have loved it.
favorite scene: tie between the blood test scene, and the defibrillator scene.