After thoughtful consideration, here is a history student's top-ten list, with explanations. Enjoy!
1. ABRAHAM LINCOLN - By holding together the Union in the face of Civil War and ending slavery in the United States, Lincoln saved America from being torn apart less than eighty years after its conception. He understood the importance of maintaining one nation, and was able to hold the country together with the sheer force of his will.
2. FRANKLIN D. ROOSEVELT - When FDR assumed the office of the President, the nation was in the throes of the Great Depression. His optimism and ability to try new things to move the country forward prevented America's decline into dicatatorship (which many at the time were calling for to relieve some of the suffering and move the economy), and saved our principles of liberty and freedom. Along the way, he introduced social safety net programs like Social Security to protect people from suffering abject proverty. His leadership in World War II - setting lofty goals for American achievement in production, homefront conservation, and military success - helped us defeat fascism around the world.
3. THOMAS JEFFERSON - As the author of the Declaration of Independence, Jefferson's crowning gift to American history came before he was ever elected president. However, his opening up of the Louisiana Purchase is a critical chapter in American history as well; it nearly doubled the size of the United States, and gave us control of land from coast to coast.
4. GEORGE WASHINGTON - Like Jefferson, Washington's greatest achievements happened before his presidency, as commander of the Continental Army. As a president, however, George Washington's biggest gift to the nation was to establish the protocols of the presidency. He adopted the title of 'Mr. President', instead of Your Majesty, as others had suggested. He also declined to run for a third tirm, setting the precedent for self-imposed term limits (this was only enacted into law after FDR's death).
5. JOHN ADAMS - John Adams makes number five on the list for an unusual but important reason: for losing the election of 1800, and turning over power to Jefferson. This may not seem important; however, this was the first true test of the American system. When Washington stepped down, Adams - like Washington, a member of the Federalist party - became the next president, and despite a new president the same party remained in power. With Adam's loss, Jefferson (a Democratic-Republican) would take over. Instead of becoming despotic or using his powers of Commander-in-Chief to keep the office, he acquiesced to the tenets of the new Constitution. Adams was the first president to prove that true democracy in America - in a two-party system - could actually work.
6. JAMES MADISON - Madison makes number six on the list simply on his merits before becoming president - he wrote the Constitiution and most of the Federalist Papers. Enough said.
7. HARRY TRUMAN - After helping to bring World War II to a close, Truman's internationalist beliefs help to ensure the creation and success of the United Nations. He also instituted the Marshall Plan, to undertake the rebuilding of Europe after the war razed much of the continent, including Germany. (Truman would not make the same mistakes made after World War I by shamefully punishing the defeated.) In 1948, he initiated the Berlin Airlift instead of directly challenging the Soviets militarily in East Germany, thus avoiding further possible war in Europe.
8. THEODORE ROOSEVELT - A leader of the progressive movement, Roosevelt is remembered as a natualist and conservationist, responsible for the creation of many of our National Parks. He built the Panama Canal, fought for consumer protection, and reigned in the power of the railroads.
9. JOHN F. KENNEDY - Despite Kennedy's assassination, his short tenure in office did have some important successes. His handling of the Cuban Missile Crisis prevented globar nuclear war. His challenge to America 'to put a man on the moon by the end of the decade' spawned a new interest in science and math within American youth that led not only to winning the moon race, but making so many of the technological leaps in the past forty years.
10. ANDREW JACKSON - Jackson, before being president, was the hero of the Battle of New Orleans during the War of 1812. As a president, he founded the first national bank (and is remembered for it on the $20 bill). While many lists put Jackson higher, I would prefer not to reward him with the honor due to his horrible treatment of Native Americans. Despite the Cherokee Nation winning their suit to remain in Georgia from the Supreme Court, Jackson disobeyed the ruling and sent them west (the Trail of Tears) anyway, saying that the Supreme Court didn't have any army.