8. The Swinging Corpse - Outlast
You know a game is supremely effective at horror when it's pretty much screaming, "You will get scared playing this" and still manages to catch you off guard.
Case in point: The early-going of Outlast, which is a masterclass in how to build tension. First you spy a silhouette in the window of the asylum you're investigating, then after scrambling into a vent, an enemy walks into the room you just left.
You breathe a sigh of relief and slowly press forward. You go through one door which should have something behind it. but nope. Nothing.
You breathe another sigh of relief and continue, and THAT is when a whopping great corpse that's been strung up to the roof comes swinging right into view.
You scream, you yell, you call out, but nothing's there. There are no enemies, no followup, developers Red Barrels just wanted to know they could mess with your head. From here on out you'd never approach any room the same way again, and it's an atmosphere that makes Outlast insanely effective.