The zombies from the Shalebridge Cradle in Thief Deadly Shadows. My friend always went on about this level being the scariest he'd ever played, so I bought the trilogy. It didnt have the same effect on me, since he told me that the first half of the level has no one in it, so it wasn't as creepy since I already knew.
However, they were still terrifying. The way they make no sound themselves, but instead make all the lights flicker where they are, and the way they spasm and move around. Also, knowing the story behind the Cradle and who the zombie were also added to the complete and utter terror I felt wandering around.
I genuinely screamed when, while I was happily picklocking a door, I turn around and see a zombie charging towards me in complete silence. I spammed flashbombs and just ran.
Some things are scarier when they don't make a sound.