That is an impossible question to answer. No one even knows which will be the most powerful in ten years.
The U.S. became most powerful because of RELATIVE economic and civil freedom as compared with the rest of the world, and because of control of resources. It is no longer any better than any other country, so the title of "king of the hill" is up for grabs
Naturally, Americans (the U.S.) think they will be (remain) most powerful.
Naturally, the Russians think they will be.
Naturally, Chinese think they will be.
Naturally, Indians think they will be.
Naturally, Brazilians think they will be.
My w.a.g. of France or Italy makes just as much sense.