Zombies. I don't even have to think about this one. Something about a foul,, re-animated, rotting corpse trying to take a chunk out of you, would scare the living **** out of me. Whether or not they run or shamble makes no difference. The Undead horde would most likely overwhelm you by sheer weight of numbers alone, to say nothing of having to smell them as their putrescent bulk closed over you. Then to feel decaying teeth clamp down on your living flesh...*shudder*! What could be scarier?