Well it used to be The Grudge for a seriously long time (I saw it when I was about 10 and had nightmares, nearly literally pooped myself. But then I saw the Descent and I realised that everybody has one thing that scares them more than anything else, call it a primal fear. Maybe there's a DNA memory Assassins Creed-style and somewhere in our genetic history somebody had an experience and the fear carries on. But I'm getting away from what I was saying, The Descent scares me because of crawling, subhuman, cave-dwelling monsters.