The worst?
1. James Buchannan - for allowing Union capabilities and positioning to erode in the lead-up to the Civil War
2. Donald Trump - for breaking a long-standing tradition of peaceful transfer of power and for inciting the Insurrection of January 6, 2021
3. Richard Nixon - for allowing the country to tear itself apart over his unconscionable behavior related to Watergate
4. Jimmy Carter - for the shame of the 444 day-long Iranian Hostage Crisis and for the 21%+ Inflation Rate during his administration
5. Ulysses S. Grant - for the corruption and scandal associated with his administration
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Mind you, I'd trust the word of Jimmy Carter over that of damned-near any other President in US history; it's just that he was a bad President.
And I and the nation are grateful to Ulysses S. Grant for his persistence in bringing the Civil War to a close; it's just that he was a bad President.