The short list of America’s greatest presidents is one that most historians agree on. Many books have been written about George Washington, Abraham Lincoln and Franklin D. Roosevelt, the three men who top most any list on the topic. But what about America’s worst presidents? In his new book, journalist Robert Strauss examines history to come up with quite a few candidates. But one man takes top billing when it comes to the bottom: James Buchanan, a Democrat who served as the 15th president from 1857 to 1861. Strauss explains why Buchanan left a lousy legacy in the Oval Office in his book, Worst. President. Ever.: James Buchanan, the POTUS Rating Game, and the Legacy of the Least of the Lesser Presidents. He spoke on the Knowledge at Wharton show, which airs on Wharton Business Radio, SiriusXM Channel 111. An edited transcript of the conversation follows.…