I can't decide between Basic Campfire, Big Brown Sack or Barrens Chat
Of course Lich King will win cuz nostalgia. He was nothing, but a lame scooby doo villan with a last sec retcon to his entire premise.
The biggest wow villan in reality is nostalgia
Definitely Gul'Dan. He was always power hungry and evil even before the Legion influenced the Horde, there were no outside influences to make him the way he was. Gul'Dan chose to be evil to the core and even helped the Legion corrupt the Horde. The Lich King is obviously the most visible and known villain dating all the way back to WC3 and running up until WOTLK. Arthas was duped and led down a dark path and ultimately corrupted by Frostmourne. Garrosh was also a compelling villain with a lust for power that I'd place second to Gul'Dan.