Amnesia: The Dark Descent, but there's always room for improvement. Frictional's Thomas Grip has compiled a list of 10 items that can "take horror games to the next level," and (surprisingly) it doesn't begin with "Everything opposite of Resident Evil: Operation Raccoon City." It does, however, touch on a few points that could have put the "horror" back into that particular survival-horror title.
Grip suggests "minimal combat," "long build-up" and "doubt" are essential to evolve the horror genre, and we've already seen these aspects work wonderfully in Amnesia. Another aspect Grip notes is "no enemies," which he clarifies as follows: "What I mean is that we need to stop thinking of any creatures that we put into the game as 'enemies.' The word enemy makes us think about war and physical conflict, which is really not the focus in a horror game."
Grip's list seems to focus on different approaches to immersion and connecting to players on an emotional, human level before ensuring they won't be able to sleep properly for a few weeks at least. All 10 points are described in full on Frictional's blog.
Perhaps we'll see a few of these fleshed out in Amnesia: A Machine For Pigs, a sequel in development by thechineseroom and overseen by Frictional itself.