Based on an Internet listing of scariest movies ever I browsed a few years ago, I bought and watched "
Event Horizon" (1997; has Laurence Fishburne and Sam Neill in it).
I was not disappointed. My GF is STILL pissed I talked her into watching it. I think it definitely ranks in my Top 3 Scariest. Maybe No. 1. I'd like to watch it again, but I don't want to watch it alone and my GF still shudders and tells me to watch it by myself when I suggest we take it off the DVD rack for another viewing.

I need to find another unsuspecting victim to watch it with. Hmmm.... Tried to get GF's sister to watch it, but the GF keeps warning her not to.
"
Room 1408" (2007, based on a Steven King story, has John Cusak in the lead role) I found to be pretty damned scary
Those are the top two that come to mind.
I found "White Noise" to be ... disturbing.
A movie that may still be in theaters in your area that isn't quite as scary as the two at the top of this post but I found very entertaining and appropriate for the Ghostmas Season is "Jennifer's Body."
Not really all that scary (my GF didn't watch this one through the crack between her fingers), but I liked it.
Critics didn't, so.... (shrug).
Oh. Don't go expecting to see Megan's Body because you don't get to. That was just tricky marketing. (SNL skit on Megan's "nude" Internet pics btw, was kinda funny ...)