I've played few horror games that genuinely frightened me, though F.E.A.R and the expansion packs definitely had moments that made me jump - in fact, one area where the expansion packs are better than the original game is the frequency and quality of the scares.
Still, I've often wondered myself why, no matter how intricate, evil and disturbing a game's setting or atmosphere is, no game has managed to be consistently and genuinely terrifying. I'd have to answer that it's the gameplay itself that is to blame. The early Resident Evils relied on jump tactics, and the sense of helplessness was more the sluggish, imprecise controls and inclusion of an absolutely useless knife weapon than anything else. As for Resident Evil 4, that's about as scary as a labrador puppy. Silent Hill suffered the same fate of wonky controls, despite having some deeply disturbing storyline topics, but that too failed in the scare department as far as I'm concerned. Yahtzee hit the FEAR series right on the head with his review, stating that nothing's too scary when you've got a full arsenal of weaponry to guide you, and the shooting and scare sections occur with an almost audible clunk. And Condemned's scares were once again, jump tactics, and the horror element seem to be more about violence and brutality than genuine psychological threat. Bioshock (not typically a horror game, but scary to some) made the almost unforgivable mistake of having absolutely no long-term punishment for deaths, although made the best decision in horror so far by taking things into the first person.
Which brings me to my personal contender for scariest game ever, System Shock 2. It made you jump without relying too heavily on contrived set-pieces, the weapons would break with overuse, not everything was useful to you as your character only levelled up on certain skills, and nailed the sound effects - by far the most important aspect in a horror game - to great effect.
I always have been, and am always going to be a keen follower of horror games, as they do have a distinct feel to them and I'm sure one day, a developer will nail it perfectly, but for the time being, it seems developers are scared to take chances with the genre in order to create a game that will be renowned for its sheer terror factor. I'm hoping the new Alone in the Dark hits all the right buttons - I doubt it'll be the ultimate brown trousers gaming experience, but I must admit, it's looking incredible.