The Weeping Angels
You can't fight them, you can't see them! Once you know what's coming it's already done to you, you're already gone. Stranded in a strange time where you have to readjust yourself into an old society. I mean if I Weeping Angel got to me say right before Bill Gates could even think of Microsoft, I'd probably be ok, lol. That is if I was able to get the money up to buy stock in MSFT.
But other than that, their real asset to being scary to the audience is the helplessness. You don't know what's happened to you until it's already done and you're already gone. Blink! is one of my favorite episodes, it was well done, the whole thing about the transcript and the Easter eggs is brilliant.