It's somewhat a matter of opinion, since many people have their favorites and it always looks good if you answer "Nelson Mandela". The truth is that after WW 2, the world got divided into a "Western" bloc under US leadership and a Communist bloc under the Soviet Union. The various US Presidents and Communist Party Secretaries all shaped the world in the Cold War era. A good candidate however is Soviet party leader Mikhail Gorbatchev, who practically single-handedly dismantled the old Soviet Union and thereby ended Communist rule over many eastern European countries. Americans tend to thank Ronald Reagan for that, but that is vastly overrating his actual influence on affairs. We Westerners like Gorbatchev for that, but in Russia he is generally seen as a weak leader and the man who 'sold out' Russia's global power and prestige and who was responsible for much of the following unrest until "thank God" Vladimir Putin took firm control and put Russia back on the map again.
