Illithid
Illithids are cool; and by cool, I mean totally sweet. They look like Cthulu, wear purple robes with high collars and eat brains. Plus they all have psionic powers and are so super-smart even their breakfast is JUST AS PLANNED! They would be totally Mary Sue, except for the fact that they are so brain-damagingly evil it makes them awesome instead of lame.
The Illithid is considered "Product Identity" by Wizards of the Coast and as such is not released under its Open Gaming License. However, "half-cthulhu creeps" and "squidface brainsuckers" are totally okay, so go nuts.