If you could only pick one, who would you choose? Only NPCs, no level cap.

My choice would be a Volkihar Master Vampire. They can do some work. Aggressive, powerful casters with two elements, you can find them with Orcish Armor, and they have 98 skill in 1-handed. The most versatile thrall out there, IMO.

I would pick General Tullius so I can show the imperials that I mean business once I had turned their general into a mindless thrall.

Nazeem. It actually makes him smarter!