The one who managed to take down the emperor. (Not Amaund Motierre. The one who ordered and directed him to Skyrim, which I think that is the High Chancellor. I have a.... project(I write, sometimes), and I strongly support this "theory)
Or a Thalmor guy.
EDIT: Oh, and I believe that the High Chancellor is not the bad guy. Motierre says that you actually helped in someway the Empire by killing the Emperor.
Or, the High Chancellor could be a puppet of the Thalmor...
Are you asking him to be pitted against dragonborn, or other NPCs? I think if Harkon was pitted against any Greybeard he'd won - he can't be paralyzed with Ice Form, ragdolled with Unrelenting Force and any damage done by elemental shouts will be negated with his Blood Drain spell.