well the country that ended the war on the best note was the US...they had a prooven military...and had shown the world it's industrial strength, as well as having the most powerful weapon in the world! the USSR takes second place because they also had a prooven military that was willing to endure tremendous losses for victory and ended up with the most territory in the end...plus the largest land army EVER!
Then after the USSR developed it's own A-bomb it was the US and USSR trying to fight for who was most powerful (IE...the Cold war)
But right at the end of the war the US was most powerful with the USSr in a close second...at least thats my opinion.