This is difficult. On the one hand, I'm tempted to give it to the Psijics just because they're basically uber-mages and can more or less do what they want. On the other hand, the Greybeards have enough power to shake the entire province of Skyrim just by saying a name.
In the end, I'd give it to the Greybeards. There's really very little that magic can do that cannot be done through Shouting, and there has to be a reason that it was Shouts and not magic that enabled mortals to overthrow their dragon overlords. Mortals had magic all along, and the dragons steamrolled them. A dragon without Shouts is just a big flying lizard, so it had to be the Thu'um that let them rule over mortal mages (like Dragon Priests).
sijic monk.
