Oh tough question but if I had to choose:
1) Thomas Jefferson: he expanded America by more than 50% with the Louisiana Purchase, co-writer of the Declaration of Independence and oddly enough advocate for fair treatment of Africans. He freed his slaves on his death-bed.
2) Abraham Lincoln: leader of the Union during the American Civil War and although he did not approve of the freedom of slaves to be released in our country (he bought land in Africa, known as Liberia today) he wanted them released back in Africa fearing they could not fit into our culture, he still was the abolitionist President.
3) Martin Luther King Jr.: without a doubt, the most powerful civil rights leader the United States has ever seen that has advocated a policy of pacifism, unlike Malcolm X who wanted to achieve equality even through violence. MLK lead the way and became a martyr for equality of the races in the United States and still is a name used in reverence to this day.
4) George Washington: though a terrible military commander most of his life with screw-ups like Fort Necessity during the French and Indian War and the first 3 years of the American Revolution the man was a beacon of hope for the very existence of the United States. He was offered great power after the freedom of the US yet refused and on his death-bed proved to be a very wise man in saying "Beware of entangling alliances" warning us of the years to come and basically predicted we'd end up in things like WWI and II.
5) Alexander Hamilton: he may have only been the Secretary of the Treasury for many years but during the waning years of George Washington and all of John Adam's presidency he was the driving force behind any work that got done by an unsure and still heavily imperfect American government. He revolutionized the banking system and is famous to this day for his victory in a duel against, the Vice President of the time, Aaron Burr.
