The fast headcrab zombies from hl2:
It was late at night and I was completely alone in a very large (and 100+ year old [read: creaky]) house, had just sawed my way through dozens of regular zombies with nothing but the grav gun and crowbar, when I hear a godawful screech, and see this beast lurching horribly towards me at inhuman speeds.
It's even creepier when you look at it up close, and you see the muscles and bones, where the flesh was literally torn from the still-living body, likely by the person's
own hands, all while they're completely unable to control themselves; only play a horrid part in their own torture, like a puppet who's master wants only to see them suffer.
Also the screams that the normal zombies make when they're on fire are pretty gruesome as well.
P.S. To top it off, the next night I was meeting my girlfriend after work, and on the way, under a burned out lamp post (almost pitch black), I heard the
exact screech of a fast zombie. Suffice to say the walk home was more expedient.