This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Shadow Man
The Playrooms no doubt freaked out anyone and everyone who played the game as it combined innocent infant bedrooms with blood-stained killing rooms while the laughing and crying of babies mixed in with the combo soundtrack of a music box and the sounds of some kind of power tool cutting into something wet and crunchy. The Playrooms area was pure psychosis in video game form.
Most of the aforementioned rooms have their own background music. None of these can be considered a relief from the area's main theme. Here's all of them together, in an extended cut from composer Tim Haywood.
The lairs of the Five weren't much better, including a derelict, abandoned New York apartment building full of corpses and an attic filled with enough blood to form literal pools and waterfalls.
In a darkened section of the tenements is one wall with graffiti of a woman, entirely in silhouette except for a pair of dripping, bleeding eyes. Having this fucker pop into view as you frantically scan the area with your flashlight is easily one of the most heart-stopping moments in the game.
The underground London subways in which Jack The Ripper lives.
The music that plays in Jack's shrine (it's a heavily distorted and backmasked version of "Moonlight Sonata"'s first movement) is probably the most disturbing tune in the game, and considering how pants-wetting most of Tim Haywood's soundtrack is, that's quite a mouthful.
The Texas prison where all the guards and most inmates have been killed and reanimated as headless zombies where a disco-obsessed nutball, a Vietnam war veteran and an insane doctor reside.
Even the more mundane areas usually have some creepy factor to them.
The final villain just happens to be the very same "Legion" that Jesus encountered in the Bible. He's a lot scarier here than he was back then.
The sequel, however, seemed less so, as it felt more akin to typical video game dungeons. But, the main hero was now an emaciated skeleton ghoul rather than a muscle man with a mask in his chest.
There's an enemy type in Deadside that starts popping up when you enter the Asylum, all based on hulking figures in masks, who make this horrific distorted —and LOUD— piglike squealing noise when they attack you.
The Asylum level called "Playrooms" sounds like fun, doesn't it? Interestingly enough, many areas in Liveside are even scarier. It comes to a head in the dilapidated New York tenements, which features very few enemies but is drenched in terrifying, pitch-black atmosphere. The music does not help at all.
It gets even worse: the desiccated corpses in the tenement all have tape recorders playing a single two-second loop of someone whimpering in most versions, but the PC edition has them play several longer clips consisting of the Home Improvement Killer forcing his terrified victims to repeat Mark 5:9 as they sob and plead for mercy. Yeesh.
To say nothing of the walking "corpse" that emerges from a locked box in one hallway. Whoever this thing used to be, it's too far-gone to do anything but shamble around pathetically until you put it out its misery. The victim's voice on the tape deck hanging around its neck sounds even more hysterical than the previous ones you heard.
To one up the above comment, picture this scenario: You're in Deadside and you see a waterfall of blood. Normal given the circumstances of where you are. Now, back in that liveside domain you see another waterfall of blood like in Deadside and you start thinking to yourself: "Ya know what? I think Avery's killed a little more than twelve people..."
Gardelle County Jail is littered with the headless corpses of Dr. Batrachian's victims. Even Mike is freaked out when they start getting up and attacking him ("This isn't happening!"). The harsh, ragged breathing noises these victims make through their exposed throats is deeply unnerving.
Jack the Ripper's domain is also quite creepy, especially when you come upon his private chambers and find his "homework" hanging from the rafters. The fact that the bodies are concealed under bloody sheets mitigates it only slightly.
When you first encounter Marco, he starts making a disturbingly cheerful rant about the upcoming apocalypse that really gets creepy when he starts talking about Michael's dead brother, Luke. It's the tone that really sets it off...