Depends on your version of scary. Do you like the "Boo!" scary films where scary things, like monsters, are jumping out of every corner, or are you more fond of the psychologically scary movies?
I prefer the psychological ones, so I always end up recommending movies like The Ring, Fallen, and my old-school favorite Angel Heart. For "Boo!," I actually thought The Grudge was quite good. The plot was stupid, but the spirit-thing was unstoppable, and it was everywhere you turned. Very good.
At the theatres now, The Descent is getting rave reviews over at Yahoo. I've literally seen dozens of comments saying it's the best scary movie since The Ring.
On that note, I want to see The Descent, but I can't get my wife to go. She thinks it looks dumb. If anyone has seen it, I'd appreciate a review.