Andrew Jackson. He wasn't just a bad leader, he was also evil. Like, supervillain evil. Guy spent most of his military career and presidency thinking up creative ways to persecute and exterminate Native Americans, with a brief break to kick the British out of New Orleans.
Worgen said:
Andrew Jackson, he was the hitler of american presidents but the only reason he tends not to be regarded as such is because it was against brown people.
This guy gets it!
Lyndon Johnson was pretty bad too. Not a terribly BAD leader, just very inept and had zero presence, nor commanded any authority (which, really, is a common theme when vice-presidents step up). Caused Vietnam to drag out far longer than it had any right to, as well as dragging his feet on the Civil Rights issues.
Andrew Johnson. Eugh. The South is still feeling the effects of his presidency, nearly 150 years later.
U.S. Grant. Like LBJ, he wasn't bad or evil, just a bad president. Had no control over anything, drank more than he breathed, and had the most corrupt cabinet in U.S. history.
Rev. Al Sharpton. Not a president, but his constant grandstanding and vitrolic speech have really tainted the entire message behind the Civil Rights movement.