Ulysses S. Grant perennially contends for the title:
The former Union general was a drunk who handed his scandal-prone pals the keys to the country, and he was hopelessly at sea at navigating the economic upheavals of the time. When Arthur Schlesinger asked historians to rank each president from "great" to "failure" in 1948, Grant beat only the hapless Warren G. Harding, who once actually admitted, "I am not fit for this office and should never have been here."
Historian H. W. Brands, however, defends Grant:
His silence fed a feeling that he wasn’t very smart. Combined with the longstanding stories that he drank too much—stories based on actual but rare benders that never interfered with his work—he appeared to the party’s pros a man they could manipulate or maneuver around. But he hadn’t bested every general the Confederacy sent against him to be defeated by mere politicians. He declined to defer to Charles Sumner, the Massachusetts senator who had been caned to within an inch of his life by Preston Brooks in the 1850s and who fancied himself the one true guardian of African Americans. Sumner denied Grant credit for aiding the freedmen—ignoring that Grant had won the war and thereby put teeth in Lincoln’s Emancipation Proclamation—and he became a thorn in the administration’s side. …
Grant proved a better politician than his opponents, and more determined. He recognized that the only beneficiaries of the Liberals’ secession would be the Democrats, who remained beholden to the same Southern conservatives who had brought on the Civil War. Grant had never been an eager candidate, but he refused to let the Democrats win at the ballot box what they had lost on the battlefield, and he trounced the Liberals, who never forgave him.
Caption from Wiki:
Cartoon showing Ulysses S. Grant, as an acrobat, on trapeze "third term," holding on to "whiskey ring" and "Navy ring," with strap "corruption" in his mouth, holding up other acrobats, – Keppler, Joseph Ferdinand, 1838-1894