Well, you could get into a month-long debate on this question. Clearly, Jefferson was a genius and a prolific and excellent writer. One might also argue that Madison was the brightest, and they too would have a strong case. Adams - the first one - was reading Cicero in the native Latin at age 12, so he's not a intellectual lightweight. And, many literary scholars say that either Teddy Roosevelt, US Grant or Eisenhower's autobiographies are the best written by Presidents, making them candidates for the best writers.
For me personally, reading Lincoln's letter's that that have been archived, as well as the speeches he gave, leads me to believe that his economy of speech or words, measured against their power and effectiveness, make him the best writer that has ever held the office. It's a personal opinion, but one that is shared by many literary scholars.
Whoever suggested the proportional representation, knowing the North would outgrow and thus outvote the South, was the smartest.
As for Lincoln’s humor: “Better to keep your mouth shut and be thought a fool than to open it and remove all doubt,” was not only funny but good advice.
I’m not the only one who should remember it more often.