In terms of geographical position where you can exert your power and influence in wider range of the map, I vote HRE. One can achieve hegemony there without having to wage war with its bordering factions and maintain a relative peace for much of the catholic world.

I would have conquered all of Europe by now at turn 70 but thought this would give me a greater challenge. I have not as of yet fought a single battle with the France, England, Poland or Hungary. Now I see large cities sprouting and huge armies marching all over Europe for a greater finale.