At first I was going to say FDR, then Einstein, but the more I thought about it, the more I realized that we don't have to limit it to countries. There is an individual who has dominated this century so extensively that its a no brainer--there's only one guy.

If Person of the Century is defined as that person who most effected the world, whether for good or ill, then this century you have to give the nod to Karl Marx (Even if I'm not sure that he survived into the 1900's). No Really. In spite of the big Ghandi Tribute above, and the evils of Hitler familiar to all, it has been communism which has dominated the behavior and consiousness of this century more than anything else you can name. Communism and Socialism, or the fear of them has fostered more revolution, more war (particularly of the 'civil' variety), more spending, more geopolitcal conflict, in short, more human action and human change during this century than anything else you can name that can be traced to one man. You can start with the Revolutions in Russia, and China. You can point out how Hitler's rise to power came in part because he was able to pose facism as a necessary alternative to the rising threat of communism. You can move on to the Korean War, The Cuban Revolution, The Iron Curtain, The Cold War, the Prague Spring, the Solidarity Movement, The Red Scare, McCarthyism, Vietnam (which destroyed an incumbent U.S. President and shaped the political conciousness of an entire american generation). Pope John Paul II is known for several things, spiritual leader, advocate of peace, leader of a world religion, yet history will remember him for his tireless battle against (drum roll) communism. Even today, communist governments represent nexii of political conflict, whether through the passions they incite (Cuba), or the power that they wield (China), or the diplomatic difficulties they pose (North Korea).

If you lump all of 20th century technology together, you MIGHT come up with something that has had as much influence, but if you limit it to something that you can trace to one man, its got to be communism, and the man has to be Marx.