US in 1945. The UK was, in 1815, the first among equals in Europe. Prussia, Austria, and Russia were all counter-balancing powers, and the UK wasn't strong enough to fight them all.
The US in 1945 had no peer power. The USSR came closest, but couldn't exert influence outside the territory it controlled directly.
Easily the United States. The most technologically advanced military on Earth and in possession of the world's largest navy. And of course they now have the bomb.