Truman.
Sure, he appears on a lot of Top Ten lists, but I think he should be higher. I think making the decision to be the first to use nuclear weapons was one of the most important decisions in American history. I think using the nukes only twice kept America safe for half a century or so, and set the stage for the Cold War staying cold. If Truman did not use nukes to end WWII, I truly think there would have been a WWIII between America and the Soviet Union. It probably wasn't necessary to end the war, but it was a damn good decision in the long run. I think people bash him for this decision, but I think he ought to be praised for it.
And the Japanese NEVER would have surrendered otherwise. He saved countless more lives than were taken by dropping those bombs. The Japanese were ruthless.