Their best villains are sort of those not even any sort of human, the only good consistent examples I can think of are Bwon’samdi and Denathrius, also MoP’s Y’shaarj and N’zoth despite the fact that i feel he was way too overpowered for being the weakest Old god to date compared to even a dessicated Y’shaarj or Y’ogg Saron.
And I’d like to point out that when they DO write a good villain, it’s always accidental… and nearly short lived when they try and axe them off or DO in one patch.