Most terrifying? "Bob" aka the Spirit Healer. That one freaked me out the first time i met him (and the corpse run afterwards^^)
Literally everything in Duskwood. You walked on the road? Say hello to Stitches! You walked through the woods? Say hello to Worgen, Spiders, Ghouls n stuff! That zone gave me nightmares. Oh and Mor'ladim, his story line was awesome. Meeting him on the graveyard while questing... not so much.
Most dangeous? "Old" Fel Reaver/Mor'ladim, don't have to explain that^^
Honorable mention to my good friend the cleaner and every Elevator since Gnomeregan.