Most Powerful Nation

I think the US was overall the most powerful nation after the war, it had a big army, both its navy and airforce were the strongest in the world, with lots of experience gained during the war. Moreover the US economy was booming, the country itself and the civilian population were untouched, etc...
The Soviets had the largest and strongest army in the world, but their manpower reserves were severely depleted, and much of their land was devastated. Britain had a big empire left, but its economy lay in ruins. So I would have to go with the US on this one, since they had not suffered half as bad as many other nations.