not that it matters considering most of the 52 Muslim Nations dont rule by Islam but out of intrest which the most powerful Muslim Nation? In terms of Military, Intelligence, Economy, or Influence etc ?
None of the Muslim Nations can take on the mountain task of Uniting the Ummah on their own.
The best way to think of the Ummah is to forget the poison of Nationalism and think of the Ummah as whole Land-Mass geographically.
Example USA: When the 51 states of USA are united they rule the World but if USA was to become 51 different states with puppet monarchs and tyrant rulers their glory would quickly crumble.