Who Was Worst?

Another presidential poll has come and gone, this one populated by political scientists, who tend to measure presidents according to how closely they approximate their conception of the modern presidency (thus historians tend to rate Ulysses S. Grant higher than do political scientists). But the folks at the bottom include three Civil War-era names: Franklin Pierce, James Buchanan, and Andrew Johnson.

Which one do you think was worst … and why? Often the criteria one uses is critical to understanding these assessments.