It truly is hard to say who will be the most powerful faction, at least until the game is finally released.

But ultimatly, the most powerful comes from being the faction that can best manage their economy and also must gain control of strategic territories that will aid in their defense.

The value of the money is self explanatory, can't afford an army worth anything without being able to pay for it, whether self built, or hired out as mercenaries.

The strategic territory will come from 1: location, an easy to defend point, or able to send troops to aid provinces/armies in trouble with ease. and 2: Providing unique resource/economic advantages to the owner that allow for more $$ or better armor/weapons.

Whoever can gain control of the $$ and the proper provinces will have the advantage when seeking to control the world.