I voted Killed By Death, because I remember being completely freaked by it, even though I haven't seen it in a long time. I considered Hush, because, well, The Gentlemen and that song the little girl sings in the beginning, but I appreciate Hush more for the way it was made than how scary it was. Killed By Death isn't as overall impressive, it's just creepy.
I do agree about Dawn's scenes in Conversations With Dead People though. I fell asleep during that episode once (it was very early in the morning), and woke up again when things were flying everywhere and Dawn was screaming, I was seriously confused and a bit freaked.