I think 'the decent' was bloody scary! I couldn't think of a more horrible nightmare situation to be in apart from maybe an open water type situation...
The saw movies were pretty scary. I think I seen just the first 2. Those movies are sick and not in a good way.
The scariest movie I ever saw was the original "The Thing" in theaters when it first came out. I think I was 7 at the time and it scarred the crap out of me.
There are so many that are close to the top of my list. I think it's a toss up between Psycho and Silence of the Lambs. The blood and guts type including the Super Natural types of scary movies don't scare me much. It's the more realistic scary movies that get to me. Two of the newer ones that weren't block busters but still pretty good were Wolf Creek and Shuttle.