There has been a challenge recently, following the BBC's compilation of a list of the "ten worst Britons", to name the ten worst Americans. Most participants, however, have compiled their lists without regard to chronology, which is very different from the BBC's one Briton per century. The American equivalent is, I think, to divide the time since 1755 into 25-year blocks. In each of these, we hope to find one of America's worst.
I am biased towards those who made a broad negative impact, rather than the Jeffrey Dahmers of the past.
1755-1780: Benedict Arnold is the only name that comes to mind, though his treason did less harm than Horatio Gates's incompetence.
1780-1805: Aaron Burr laps the field: Vice President, killer, and traitor.
1805-1830: erm, I don't have a good candidate here.
1830-1855: John C. Calhoun, resigned from national office to become the foremost defender of slavery.
1855-1880: Nathan Bedford Forrest, Confederate General and KKK founder, seems to have the strongest case. Some have argued for Jefferson Davis, who I think was mostly a functionary.
1880-1905: William Jennings Bryan, perhaps, for his indefatigable work for the wrong causes. He was for imperialism before he was against it!
1905-1930: Alphonse Capone, greatest of the crime lords and the one who did the most to pervert the courses of justice and democracy.
1930-1955: J. Edgar Hoover needs no introduction.
1955-1980: Ken Goodman, who (with Frank Smith) popularized the "whole language" movement and bequeathed to us half a generation of illiterates. Or we could bring back Mr. Hoover for a reprise, opening the World War II slot to a contender like Axis Sally.
1980-2005: "Pat" Robertson, the archetypal idiotarian, who has systematically inflamed the culture wars and done more than anyone to discredit the very idea of moral decision-making. An idiot useful to extremists of both sides; a malevolent tool.