Associate
A few of the top of my head, the shambler from quake, the controllers from stalker and that enemy you had to freeze in RE4.
Have only one round of ammo left in you shot gun trying to escape the library in metro 2033, when your surrounded by Librarians mutants.
your hidden in the cupboard and he's raping one of the four-legged nurse things.
Pyramid Head absolutely **** me up in Silent Hill 2, the bit where your hidden in the cupboard and he's raping one of the four-legged nurse things. Scary stuff.
Oh.. forgot the inmates in The Cradle in Thief 3. I had trouble playing that level after dark.
Anything in Demon's Souls world 5 and in fact the world itself.
Hell even thinking about that place makes me uncomfortable.