The designation of “America’s Greatest President” is subjective to be sure. The public and historians often disagree when it comes to the subject of the Greatest American President. The wikipedia entry on this subject is rather thorough. Generally, the following five presidents are in the Top Five of most polls:
George Washington
Abraham Lincoln
Franklin D. Roosevelt
Thomas Jefferson and
Theodore Roosevelt.
Ronald Reagan and John F. Kennedy tend to score highly in popular opinion polls.
Wilson and Truman do well in those rankings, too.
You may find this recent NYT article, “The Myth of the One-Term Wonder”, interesting:
http://www.nytimes.com/2010/02/14/opinion/14merry.html