This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Condemned
Black Lake Lodge from Condemned 2: Bloodshot is arguably the scariest level in the entire game. You need to investigate a dilapidated hunting lodge in the mountains that's been abandoned for decades. Dead bodies. Rabies. A guy being mauled by a huge, rabid Grizzly Bear with blood caked to his fur and strings of drool hanging from his jaw. Being chased by the huge, rabid Grizzly bear. The huge rabid, unkillable Grizzly Bear. Oh, just a touch from him and you're treated to an extremely horrifying "game over" screen featuring the bear knocking you to the ground, standing on his haunches and roaring, mauling you and finally literally biting your face off or ripping your arm off.
The nightmares don't stop there. The meat of this level is searching for detonators in the main lodge while occasionally fighting SCU members sent to "clean up" some crucial evidence and kill you. It's unnerving enough to know that you're being hunted, but if you value your pleasant dreams, then please, for the love of God, DO NOT look closely at the taxidermy scattered throughout the lodge. The next level, taking place in both an abandoned bowling alley and the school from the first game, somehow came as a relief.
In the apartments, there's a part where you enter a bathroom. Everything is quiet, then you grab the medkit next to the mirror, and what do you see? A freaking enemy in the mirror.
The Mall Level in Condemned: Criminal Origins. The living mannequins, dilapidated interior, mysterious deaths, the dead body down the escalator,the weird warped Christmas music playing incessantly, and finally the matchmaker killer you was supposedly chasing is found dead, posed, and killed by his own methods.
The abandoned school level. Those little imp-like people, the creepy, messed up Lunch Lady, and the final scare in the form of the torturer's last victim found in a locker, supposedly dead. Then Rosa HAS to ask for a closeup.... Its made worse by the fact that you're expecting it to happen...
He cut off his victim's goddamn lips!!!
Also the part where you come across the dead body of yourself.
The underground tunnels from the first game. They're populated by bone-white, anorexic looking psychos, who appear to be teenagers, who'll try to kill you regardless; it's cramped and attacks can come from all sides as well, and if you go far enough, you'll face giant axe or shovel wielding psychos in firefighter coats.
The basement in the Library level. It's burnt out, smoky, highly claustrophobic, and constantly has bizarre mutants reaching through the bookshelves at you while you run through the thin areas between the stacks.
In the first installment, when you reach the farm house within the apple orchard and you must walk through the seemingly empty house. Complicating things is the fact that for the most part you are without your flashlight because you need to have the blacklight on to see the trails of psycho babble written on the ceiling and floors. It's even worse because after an entire game of being attacked by psychotic hobos and plagued with mind screwing hallucinations the first and second floors of the house are completely empty albeit very disheveled and creaky. Then when you finally go down to the basement a couple of the pale junkies pop out and rush you.
Then there's the moment you leave the basement only to discover you're not alone anymore...
In the second game in the abandoned doll factory it was those fucking dolls...it's not just the way they look that was scary but the fact that they were ''alive, you can use them as weapons and if you hold it in front of you, the thing still moves.
The apartment complex in the second game features a room with a posed mannequin in the centre. Go through the doorway on the right and turn around. Well how about that, three more mannequins have been moved (or just, y'know, moved)to block the way back. This becomes something of a pattern for a while. Considering that the first game had the exact same mannequins stalking and surrounding the player in the department store, this hallucination seems to exist purely to cause PTSD-style flashbacks in the player.
In the last few minutes of the first game Ethan freaks out and seems to transform into some kind of deranged monster. Then he roars at the camera. The end.
You think being attacked by a psycho disguised as a mannequin is bad enough? Wait until you see what they look like beneath their disguises.
In the subway level of Criminal Origins, you open a cupboard and get a medkit. You open the cupboard next to it-and a crazy zombie-like lady jumps out and smacks you with a rebar.
During the apartment level of Bloodshot, you find a message in blood warning you to stay away from a certain room. If you ignore the message and go in anyway, a thug with a shotgun barricades you in with a mattress and starts firing at you. What makes it scary is that at this point in the game, firearms do the same amount of damage to the player as to the enemies - which means you will go down in two shots if you don't react fast enough. And when you're finished dealing with the gunman, a massive junkie bursts through the wall and attacks you.
At the climax of the first game, the player is stripped of their flashlight and taser and forced to trek through a dark orchard and surrounding farm buildings, wielding only a flaming 2x4 that acts as their primary light source. And throughout this experience, you are attacked by six foot tall burn victims wielding the same weapons that can be seen moving through the darkness towards the player from a distance, as well as the stick-wielding, pale-white, ninja-like enemies with metal devices attached to their jaw. Definitely one of the scariest segments of the series.
At the beginning of Bloodshot, the player is forced to trek through several rooms of an old medical storage building that are infested with the strange ooze monsters and the black inky substance that lines the walls and floor of the area. As you trek through these claustrophobic corridors, watching the ceiling for the sacks the monsters ambush you from, you are also forced to deal with an infinitely respawning group of monsters that can take you down in moments.
YMMV on this, but for me one of the creepiest was the revisit to the school in the second game. If for nothing else, the Torture Cellar you fall into appears to be in the same location of the bathroom you have the freak out in the first game. Just what is going on there?