Any aggro is scary to me. D: But in terms of looks.. hmm.. none of them looks "scary" yet, but ugliest are ratmen and boars. But I still haven't seen a lot of mobs yet and I believe they only get uglier at higher levels so I'm going to change my mind on that soon. xD
I think I'll find the spider mobs really scary. I hate spiders. Ew.
You can find Sand Ratman at Goblin camp. and other ratmen are found at KQ.
I think the Grave Diggers are the scariest to me. Think about it! If a big, burlesque man wielding a pickaxe charges you, won't you be kinda scared? Plus, they huff cave potions!