John Hawkins challenges other bloggers to come up with their own list of top 20 Americans. Off the top of my head, here is my unranked list
Martin Luther King
Franklin D. Roosevelt
I'm sure I've forgotten some people and there are some people on the list that I'm a bit ambivalent about. For example, I'm putting Jefferson on for the Declaration of Independence. I think he's vastly overrated as a president and is partially responsible for the vicious partisan atmosphere of the 1790s. A lot of his political instincts were so wacky that my students just giggle reading his paeans of praise to the agrarian yeoman farmer and his deep suspicions of the federal government until he became president. However, the Declaration of Independence with the Louisiana Purchase thrown in elevate him to my list. Henry Ford was an SOB personally and a vicious anti-Semite and didn't invent the car, but popularized it. He's there, as are the Wright Brothers, as symbols of the impact that the inventions associated with them had on the country.
Anyways, check out the two polls John Hawkins did of the greatest Americans according to bloggers of the left and the right.
UPDATE: John in Carolina analyzes the differences between the Right's and Left's lists at RWN.