In a world which physical incapacity is no object, I'd go with Lincoln, finished out his second term and getting another. I think even the completion of his second term it would have changed America for the better in dramatic ways.
That being said, neither he, FDR or Reagan were physically capable of extra terms. Washington did the country immense good by choosing not to seek another term. Clinton is a nothing special pol, and another term would have been a zero sum exercise. That leaves Jefferson by default.
