Let's see. I reckoned Cao Cao was a pretty cunning dictator. He controlled the Emperor without having to seize the Mandate of Heaven and used a huge army wisely to keep everything in his favour until he died.

Akenhaten. He changed the religion of ancient Egypt from the sect of Amun which had been the only religion for thousands and thousands of years into a religion where there was only one god instead of dozens like in the old religion. He even changed his name to comply with the name of the god he created, Aten. To change the religious beliefs of all Egpytians, one of the most religious societies in the ancient world was pretty damn good. Even though it was changed back after he died.