This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Resident Evil: The Umbrella Chronicles
The Radio Chatter from HUNK's level. Among other things, you hear demands from a police woman as she tries to regroup survivors, the last broadcasts from a radio station being surrounded by zombies, ads for Umbrella medicinal products, the desperate pleading of an injured, trapped survivor of HUNK's Alpha Team, and, perhaps the creepiest, a nameless man's psychotic breakdown, with fits of insane laughter and weeping assertions of the hopelessness of the situation.
IT'S ALL OVER, MAN! WE'RE SCREEEEEEEEEEEEEWED!!HAHAHAHAHAHAHAHAHAHAHAHAHA—* click*
One woman can be heard saying "Freeze! Don't come any closer!", afterwards you hear her screaming as she is likely eaten.
The licker hallway is worse too. It's just a huge bloodbath now, literally.
The Black Tiger spider. They redesigned it from a tarantula reskin to resemble funnelwebs. Rebecca has the displeasure of fighting two of them face to face in Umbrella Chronicles.
In Darkside Chronicles has Leon legging it when he faces two spiders, who had again gone through a mild redesign, crawling along the walls towards him.
Colonel Sergei Vladimir. His introduction leads one to charitably think he's insane, and that's before he starts cutting himself, and his One-Winged Angel form looks... off.
Off is putting it mildly. His arms have fused together into one long, flailing tentacle with a sort of ribcage-claw at the end of it, his veins have grown through his skin to wrap around him, and his jaw is distended by a huge, maggot-filled tumor that has grown from it. After you beat him, you get a file from Wesker noting that most humans who transform because of the t-virus are mutating on a psychoactive basis — that is, the resultant abomination is shaped by the victim's mentality and thoughts. Think about what that says about Vladimir's sanity that this is what he turns into?!
The creepy OST music that plays during the bonus "Nightmare" level on Umbrella Chronicles. Try listening to that with the lights off before going to sleep. Sweet dreams!