Anyway, back to the original topic....

If we're referring to the game, then I agree with satchef1 that the Byzantines should be the most powerful faction, especially in the early part of the medieval period. They were ridiculously wealthy, owing largely to the trade and tolls that passed through Constantinople--and indeed, the "Big C" raked in copious amounts of cash in MTW once you had trade routes set up.

In addition, if their unit roster in Medieval 2 is as varied as it was in the original game, then the Byz should be well-equipped to deal with their foes on the battlefield. I didn't find their lack of gunpowder units to be much of a handicap, and you could always hire cannon crews as mercenaries in any case. Their only real tactical definciency was that they didn't have any heavy spear/pike units to counter heavy cavalry--one had to rely on either Varangian Guard units (which are very expensive), and/or Byzantine Infantry (which are pretty cheap, but require significant valour/morale upgrades to withstand heavy cav). I'm hoping that Medieval 2 will give the Byz a better anti-cavarly unit, although I'm not sure how historically accurate that would be.