FDR, He is the closesed think to a monarch the U.S. has ever seen. He could of easily been elected again if he was still alive. The U.S. at the time was the largest industrial power in the world.
Actually, I would more say Harry Truman in 1946. That's the time when the US power has reached its peak. Europe was in dust, economically backward, Russia was still a country of desperately poor farmers (it's only in the 50's that the Soviet Union started to get really industrialized), Asia was still undevelopped and Japan was in ruins. In all this, Truman ruled the only nuclear power of the time, and he's the only ruler who ever used it against an ennemy.
Hitler would be close, he would of won the war if he just listened to the generals during the war. He would of not declared war on Russia, would of not declared war on the US, and had lots of troops on Normandy during the d-day landings. This ability to take over Europe made him very powerful
What if... what if... what if. Frankly the war between Hitler and Stalin was unavoidable. It was a matter of time. Hitler has simply rushed it because he thought that the later it would start, the harder Stalin would be to defeat. Something which isn't totally wrong.
Anyway, to your information, 1943 is the year when Hitler has been stopped in Stalingrad. It's the year of his downturn. His best year is probably 1941. But even then, Hitler had simply too many powerful ennemies to be considered as the most powerful person ever. Don't forget that during the whole war, Germany has never been the leading world power, the US were already the first producer, and the richest economy.
Actually that's the main reason why both Churchill in Britain and De Gaulle in France remained that confident about Hitler's fate. Even before Germany declared war to Russia, they knew that the US were nearly sure to win the war if they would join... exactly because its production capacities were twice bigger than German's. Their only worry, and the worry of all even in invaded countries, was whether the US would intervene or not. Churchill had only to sit back on his island and wait. At the beginning, the idea of a German landing in Britain was a worrying possibility, but once Hitler had declared war on Russia, well it was nearly sure it wouldn't happen.
There were only two alternatives for Hitler :
1/ Not declaring war to the Soviet Union and trying to conqueer Britain. Even in case of success, he was weakening his Eastern border and hence tempting Stalin to take advantage of it.
2/ Declaring war to the Soviet Union and letting Britain alone. Even in case of success, he was weakening his Western border and giving the opportunity for a British/US landing.
All this to say that Hitler could top many categories, but certainly not the one of most powerful person ever.
I believe there has never been a really communist country. USSR for example was officially always "on the road towards communism". They never got there, understandably. I do not know if N.Korea or Cuba have declared "arriving" at communism, however. Anyway, that would simply be propaganda.
