Dagoth Ur, probably. I think he's mostly misunderstood, not evil. He wanted to help his people and free them from what he viewed as tyranny, even if his methods were... questionable. But hey,
you get backstabbed by your best friends and be forced to live in a volcano for thousands of years, and we'll see what you'll do. A little assassination and terrorism is to be expected of him and his followers. I also love how he's a true gentleman all they way.
Aryon of House Telvanni is also one of my favorites. He's quite similar to Dagoth, to think about it: the only mage lord who actually cares for his people and House, but utterly ruthless when it comes to destroying competition.