I think the thing with Event Horizon is that it was really good at the time it came out. Since its release, lots of other movies have come out and used some of the same tricks so the effect is kind of dulled.
Still though, I'll always remember when they watched the video log of the previous crew. Ugh. Still creeps me out.
I'd say my scariest movie is THE GRUDGE. Yeah, I know some of you are SEVERE haters, but it played on a lot of my fears. Like somebody said on the first page, it tapped into the whole ghosts, hiding under the blankets, etc. The sequel was TERRIBLE. Total cash-grab that paid no respect to the original at all.
Hostel, Wolf Creek, etc. are all just torture flicks that do nothing for me. I'm all about super-natural and unexplainable for my horror. Killers are just so boring. At least the old horror flicks with Micheal, Freddy, and Jason had killers that couldn't be stopped, y'know? Made it a little bit scarier, but not much. At least at first they all had a mystery about them.
Descent was a decent horror flick, I thought. Really good until gollum showed up, but overall much much better than most recent efforts.
High Tension sucked hard. Not even worth talking about.
Silent Hill had potential but was never scary. It was at least cool to watch though, fan or not.
Devil's Rejects was enjoyable, but never scary. Just messed up. Wouldn't call it horror.
I enjoyed Dawn of the Dead remake, but other recent remakes have all been shamefully bad (Texas chainsaw, Omen, etc.)