I don't know about scariest but I would most definitely say they're the deadliest - you know, in a hypothetical "this is really happening" kind of way. When you're in the presence of a hunter, tyrant or licker you're expecting an attack because it's deformed and quick and openly hostile but a zombie is different. Zombies mislead you into believing that you're basically in the presence of a human being and you're not expecting anything amiss, until you get too close and realize that what you're dealing with isn't necessarily human anymore.
(Great topic, by the way; very insightful. )
I agree with everything she's said. Given, one group of monsters I'd say are equally applicable and a tad scarier than zombies would be Palin groupies, but other than that she's nailed it.