Hitler 100%. He had almost the entire world against him and nearly came out on top and his ideology of national socialism is still feared(by the Jews) to this day.
Stalin almost lost to a country smaller then the size of Texas and really only won because he could send more men and guns into the meat grinder when the German's supply lines started to thin, was denounced by all of his successors, and was viewed as nothing more then a paranoid lunatic.