Harry Truman. Because he made probably the most difficult decision any man ever had to make. Whether to drop the bombs and kill a lot of people or let the war drag on and have many people die, including our boys in uniform.
That was when the Democrats were still Democrats. Roosevelt was the beginning of the transition, but Truman took them back on track for a short time.
Probably George Washington, but there are many people who stood up for their beliefs that I admire. Washington risked everything he had on a venture he thought would fail. Lincoln did the same. MLK and Gandhi set the standard for non-violent resistance, but unfortunately that only works in an open society. If Gandhi or King had been in Stalin's Russia, Mao's China, or Hitler's Germany, they would have been killed and their movements ended.
He's not historical yet but will be. It's Trump. He's taken on the entrenched career government establishment and attempted to redirect the purpose of government away from the interests of the powerful and towards that of the people, which is the reason for his current persecution.
I guess I don't admire many politicians, whether historical or not. I was thinking along the lines of Jonas Salk, Marie Curie, or maybe Willis Carrier. (Thanks, Willis. If not for you I'd have to live in Maine.)
FDR was far from perfect, but I feel that he is willfully misunderstood by both the left and right. I'd hate to see the mess we'd be in now if he hadn't kept that nutjob Churchill under at least some measure of containment. And then there's Eisenhower, fairly quiet but mainly due to being muzzled and undermined by established interests. He saw a lot in Europe on both "sides" that he was never allowed to voice publicly. Things are not what they seem and probably never will be again.
Alexander Fleming, discoverer of Penicillin. There is more to that story, but Fleming is the person associated with the discovery of Penicillin, which has saved tons of lives.