I'm not particularly a big fan of horror films that involve a lot of blood and gore. For me, the best horror films are the ones that aren't too graphic, but that shock and startle the heck out of you. Sam Raimi is a master of this! I found Boogyman to be one of the scariest films by him. I find the Japenese psychological horror films to be among some of the scariest. They are not graphic, but will startle and scare the you-know-what out of you, movies like The Grudge and The Ring. I hear One Missed Call is also supposed to be a Japanese remake which looks extremely terrifying!
I sometimes make a joke that we do for Japanese horror films what they do for our electronic appliances. They imitate our electronic appliances and improve upon them. We take their Japanese horror films, and remake them and improve upon them
This past summer, I also saw 1408 which is one of the scariest psychological horror films I've ever seen. It had me screaming in shock a few times, at the embarrassment of me and the amusement of my date
The Sixth Sense certainly has it's scary moments and has BY FAR the greatest surprise ending of any movie ever!