The Exorcist was actually really scary for a 70's movie. The House of Wax was also really good! The Saw series was more of diabolically genius and bloody opposed to scary, but i loved it. I saw the descent a few weeks ago at like 3 AM, and It started scary, but kinda got stupid in my opinion... Got good reviews though! I'd check out the exorcist, or the house of wax.
I don't actually like scary movies. Not because they scare me, but I just find the stories are always a bit rubbish!
However my girlfriend was really scared by a movie called "Fragile", but I myself found it a little lame depends on what you like. It does have some creepy parts.
Buried is a movie that really got me just because I didn't realize how claustrophobic I was until I watched it @_@
I'm a big fan of zombie flicks, and Dawn of the Dead has got to be one of my all-time favorites. Watch the old one for a classic scare, and the newer one for updated visuals that are jusssssst ugh so incredible.
27 hours is scary if only for the x minutes in the film where you're forced to watch him......
Then I liked the Quarantine films because they are a surefire way to make me doubt the fact that I am an adult with control over my body. Many a casualty have resulted from watching these movies with me.
Edit: If you like Quarantine, the Rec series is a phenomenal follow-up
It is not so much scary as it is disturbing, I could not finish the movie because it just was so weird, but House of 1000 Corpses. Not scary jump in your seat but god damn that just made my skin crawl. Another disturbing one was Human Centipede 2, not 1 because that one was a joke but Human Centipede 2 was just creepy. It was done in black and white, and just plain too much to handle. Really fucked up.