There are at least two ways to answer this question.
In one sense, Germany and Japan lost the most in this war. They were each subjected to long occupations that completely transformed their countries. They had their old governmental systems destroyed and remade and had some changes made to their social systems as well.
In another sense, though, the Soviet Union was the country that was most clearly devastated by the war. Something like 13% of its population was killed in the war and many of its cities were destroyed. This was the highest death toll of any major power in the war.
One could argue that Germany suffered the most since it was destroyed as a political entity and also lost over 10% of its population in the war.
It could be argued that Great Britain paid the heaviest price as a result of World War II. Before the war Great Britain had an empire on which the sun never set. The expense of the war bankrupted the country. They realized they could not afford to maintain the navy and army that would be needed to control such a vast empire, especially since the U.S.S.R. and Communist China would be sponsoring rebellions all over the world. The British lost virtually their entire empire and now have only a few outposts such as Gibraltar and the Falkland Islands.