Horror movies are very subjective. There are factors involved that affect how you view them... the age you were when you saw them, the environment (cinema or with lairy mates), and the genre of the horror film. Here's my 'expert' viewpoint on some of my faves...
(not in order of scariness or quality...)
REC - (handheld documentary style).. Brilliantly excecuted and totally non-Hollywood. No music, no heavily flagged shocks... just pure terror... and a final 15 mins of utter horror.
The Exorcist - (demonic possesion)... Done to death by so many inferior films, so often seen by today's teens as a bit of a joke. I find it chilling in the way the events are presented very matter of fact... disgusting, but great, effects too. Even a horrifying subliminal image to make you crap yourself.
Hostel - (torture porn)... Interesting story (quite believeable!)... but one for the gore-fans only. Quite stomach churning.
The Descent - (subterranean horrors)... Hard to watch because they are all lit by torches, but utterly claustrophobic, great all-female cast and some really horrible "things!"
The Legend of Hell House - (haunted house)... Quite scientific and interesting, with a creeepy score by Delia Derbyshire. Some utterly terrifying set pieces.
Paranormal Activity - (demonic possession)... A new entry which scared the hell out of me. Simple, effective and well shot.
and finally.... the piece de resistance....
The Haunting - (haunted house)... It has dated a bit now (made in 1963), but this black and white gem has some great performances and scary scenes using only sound and lighting to convey the horror.
Just a few, but I cannot express strongly enough how important sound is to a horror film. If you have a surround system, USE IT. If not, make sure the sound isn't too quiet.
Hope this helps

))