This is hard. I am not a fan of the personality cult of the US presidency. Since Kennedy, the first 'rock-star' president it has become increasingly a role designed for a media star. Reagan being the worst excess, an actor playing a role, without any real political or social awareness who did catastrophic damage with his simplistic view of economics.
Obama is a decent combination of both media style and intellect but has lacked strength and success so far.
In a disturbing irony, George Bush snr. who was a pretty hit and miss, right winger, was the one who got the job by working hard in the background and doing a credible job. He didn't court the media like others.
I can't pick him though, too right wing.
So I have to resort to historic cop out and pick Lincoln for his work on slavery.