I don't like how so many have listed 1964, I liked both candidates that year.
Aside from Vietnam, Lyndon Johnson was a good president.
Aside from appealing to racists (and he didn't openly appeal to them, they gravitated to him) Barry Goldwater was a good candidate who would have been a good president.
1964 is one of my favorite elections. I love looking at that sea of red counties
2 Bad Candidates Since 1900:
1908, 1920, 1928, 1980, 1984, 1988, 2000, 2004 (although Gore and Kerry would have been good presidents)