Demogorgon, Prince of Demons!
There is none mightier.
There is none fouler.
There is none who garners as much fear or loathing.
DEMOGORGON, PRINCE OF DEMONS!
Sure, Orcus controls undead, but so do half the gods out there.
Yeah, Juiblex is ugly and stinks, but so do half the PCs out there.
Graz'zt has extra digets and he's a smooth talker, but he's not nasty enough.
NO! Not one of the Demonic rulers of The Abyss can withstand the might, the menance of MUMR--- errr... DEMOGORGON, PRINCE OF DEMONS!!!!!!
Now, if we want to talk about the best overall evil guy out there, no doubt that would be Asmodeus, but since we're talking of Demons, it is
DEMOGORGON, PRINCE OF DEMONS!