Ben Quayle ad: Who is the worst US president in history?

Ben Quayle, son of former Vice President Dan Quayle and Republican contender for Arizona's third congressional district, raised his primary campaign profile this week with a provocative ad (see video below) accusing Barack Obama of being "the worst president in history."

Presidential historians often rank James Buchanan the worst American president, largely due to his weak leadership while acrimony between the North and South intensified before the Civil War. Another frequent pick is Andrew Johnson, who succeeded Abraham Lincoln with controversial Reconstruction policies.

Among modern presidents, George W. Bush is deemed by historians worst; he was ranked 39th (out of 43) in a 2010 Siena College poll.

Who do you think is the worst US president in history – and why?

Share your answers using the form below.