According to a recent opinion poll, only Nixon has a lower overall unfavorable rating than George W. Bush, among all historical U.S. Presidents. The unfavorable rating for Nixon is 60%, for George W. Bush it is 59%. But, Bush still has 18 months to further tarnish his historical legacy, so Nixon may yet leave the woodshed of historical shame.
The least well known President, John Tyler, also deserves some downgrading. He brought us Texas, failed to broker a compromise to end the Civil War, and served in the Confederate legislature. We'd have been better off without him.