I have a very hard time finding the horror movie that is genuinely frightening. Most feel far too manipulative and hint at the "man behind the mirror" (Berk mentioned Cabin in the Woods and it was a movie that utterly got this.). I don't think I'm very prone to frights of the supernatural sort and though I would very much like to have the sh*t scared out of me alas I too often sniff a fraud.
One movie I recall finding very good on a both a horror and high tension level was the excellent 1985 UK film The Descent, about spelunking women and their claustrophobic encounters with a rather unsentimental race of subterranean dwellers. Not a brilliant film, but still very well done I thought.