That's a tough one, considering there have been so many terrible administrations over the years.
Andrew Jackson has to rank up there with one of the worst in my opinion, due to his decision regarding Indian removal that lead to the "Trail of Tears" were many, many Indians died.
Woodrow Wilson and FDR were both terrible presidents in my opinion. The both assisted in the dramatic increase in the role of government that we now have in our lives.
More recently there was the 2nd Bush disaster and Obama who is bascially a 3rd term of W. We are now involved in 2 wars that were completely unnecessary to begin with and don't seem to have any end in sight. Oh... and Obama wants the government to take even more of role in directing what everyone does.
So it's really hard to pick, and there are allot of bad president's that I havn't even mentioned.