Condemned: Criminal Origins constantly surprises me with how well it retains its atmosphere well beyond your initial exposure to it, which even great older games like Silent Hill and Fatal Frame have trouble with.
Usually, once I've gone through a game once or twice, playing through it again will yield far fewer scares, if any; at that point, I become too keen on picking apart the game's design quirks and nitpicking too hard to really get creeped out anymore.
But as I was going back through Condemned: Criminal Origins to wring out the last (notoriously troublesome) achievement for kicks, I was in the Department Store level, which I hold up as one of the greatest achievements in level design ever, and partway through the level, I stop and look around and see one of the mannequins in the store slowly, silently turn its head towards me. I've been through this game at least three times before, and I've never noticed that happening before, and the thought that the mannequins had been glancing around without my noticing all this time made it even more creepy.
I didn't even realize I was getting grabbed by the game's incredible atmosphere again, but once I saw that I was hammering away at the thing with my length of pipe, yelling, "No! No! No!"
It really is an amazing game. It's a damn shame what the sequel did to its legacy.