Just throwing one out in case anyone wants to join in. Name your Monster/Killer/Entity from any Creepypasta Series and perhaps why.
While it is of course difficult for me to choose between BEN and SCP-682, my money is still fondly on Mr. Welldone.
-He seems Omniscient
-Almost Schizophrenic at some points
-Enigmatic. Even by Monster Standards.
What about you guys?