Werewolves would be my favorite scary monster. I mean, who wouldn't love to be a wolf? I like the werewolves that turn fully wolf, not the half-man/half-wolf you see in the picture to your left. Though that is an awesome picture. :)
I suppose everyone has a little part of us that would love to be able to shed their human life and go feral for awhile. No human problems to plague us. Just basic animal instincts and needs. Of course, we couldn't do that forever, hence a werewolf has the best of both worlds! I don't like the movies where they portray the werewolf as dumb savage animals who only want to maim and kill though. Wolves are actually very intelligent creatures with a dynamic family life.