I would say George Washington. He could have set up a second monarchy, but declined it. He helped guide the country through establishing a decent government (as good as one can be I suppose). He is a pretty iconic figure of humility and wisdom (even if he was a mediocre general).
Abraham Lincoln made a lot of decisions that may have been avoided without 600,000+ people dying and creating a national divide that still persists today. He made a lot of executive decisions that cost lives during the war as well. He might have eased that through properly managing Reconstruction, but died (conspiracy or not) so we'll never know. He did inherit a mess, there was a series of doughface weak presidents before him that were not taking stances on any pressing social issues at that time. Even if war was inevitable he could have ended it more quickly with better judgment during the beginning half of the war (Union was consistently being stomped).
Kennedy is a hyped president, most likely because he was assassinated. He was much like the current president: young, inexperienced, and idealistic. He didn't have the experience or backbone to stand up against many of the decisions his cabinet was enacting resulting in global problems such as Bay of Pigs, Cuban Missile Crisis, Diem brother assassinations, and the Vietnam War. I actually liked many of his ideals. The more I read about him the more I saw that he didn't support most of what the US government was involving itself in, but he didn't have experience to stop it; that makes him a weaker president.