Oooh, good one! This is a really hard choice. Both scary films, both with their flaws. In the end I gave the nod to The Thing. Although the alien in the antarctic is, well, alien - the main horror is not being able to tell who's not what they seem. And that's a pretty basic fear, and the movie works as a metaphor on several levels. And was creepy as fuck, at least for the time it came out.