Gotta be the Gibbering Mouther!
You don't even want to come close this mofo. He's got all sorts of mouths of various sizes to bite the crap out of you, and even worse than that while those mouths aren't biting they're gibbering an insane cacophony. Anyone who can hear them can go nuts and attack their allies, or run headfirst into the mouther's gelatinous bulk, or just faint and wait to be devoured by hundreds of little mouths whispering their name. Ugh. Yeah, definitely the Gibbering Mouther is the creepiest of Abberations that I've ever encountered or thrown at my players. Even beating out the Otyugh (which is, let's face it, just a big bottom-feeder).