Soldato
Resident Evil Licker scared crap outta me when I was young. That and the Dogs through the window
Creeper on Minecraft. Alien AvP.
silent hill definitly has some of the most warped monsters, enough to keep them in your mind until the day you die.
All the enemies in Amnesia.
CYBERDEMON!
The controller in STALKER. It's the first and only time that I've ever completely panicked in a video game, just firing wildly in all directions. Freaked me out totally!