Over the last decade, it's been common to hear Americans of differing political persuasions claim that either George W. Bush or Obama are is 'the worst President ever'. However, people have short memories. And without any anti-American sentiment whatsoever (in many ways it's a super country)!), you guys have admittedly had some pretty good competition for terrible presidents.
I'd personally say that, economically, Calvin Coolidge was absolutely the worst. The Great Depression was largely a result of his complete ineptitude in understanding banking (particularly stocks & shares), and his presidency was one of Laissez-Faire laziness. With that said, a fairly recent biography I read of him (by David Greenberg), cast Coolidge as a rather likeable man - on a personal level, at least.
In terms of social policies, I'd nominate Franklin Pierce (the only time I would nominate him), for his bizarre, polarising policies on slavery and dislike of the Abolition movement.