Tenth Place
The Vashta Nerada
Silence in the Library/Forest of the Dead
Nobody wants to be eaten alive until there is nothing left of you but a skeleton, so the idea that a shadow could latch onto you after deciding to make you it's meal, and all you can do it wander around and wait for the inevitable is quite a chilling concept.
With the Vashta Nerada Steven Moffat manages to play on everyones fear of the dark and what might be lurking in it, only he spins it on it's head because it's actually the dark here that wants to eat your face. By making the enemy something slow so that death, rather than being immediate, becomes something that you have to wait for, it turns something that might otherwise have failed as a bad guy into something quite terrifying indeed. And it's that scare factor that wins the Vashta Nerada a place on my list!