Well Beholders and Illithids are certainly the poster childs.

Which isn't such a suprise, as the early editions cranked out a huge amount of original monsters. But somehow these two became cool while all others are still laughed at 30 years later.

Another candidate would be Githyanki. I don't think there is anything like them anywhere else.
And actually, their backstory is as stupid as the worst out there: A race of space pirates that live in a void without time and build their cities on the petrified bodies of dead gods. And their queen is a necromancer who kills everyone who could become stong enough to threaten her rule. And they have Red Dragons as pets.
But somehow, they are still cool!?