... I would say FDR in 1936, because in terms of percentage in the electoral vote, he did better than either Nixon in 1972 or Reagan in 1984 (there were only 48 states then, and the EC had only 531 electors.) 523 is a greater percentage of 531 than 520 or 525 is of 538. Plus, FDR managed to keep Landon in single digits in the EC vote.
^ This.
I will also say that 1912, with Democratic pickup winner Woodrow Wilson having unseated incumbent Republican William Howard Taft, was highly remarkable. The loss by Taft was the most disastrous unseating of an incumbent, and from the party with the realigning advantage (Rs won seven of nine election cycles from 1896 to 1928), leaving just two states (Utah and Vermont) to carry for the 27th president. Six of Taft's 1908 states (California, Michigan, Minnesota, Pennsylvania, South Dakota, and Washington), went to Republican predecessor and Progressive nominee Teddy Roosevelt. And Wilson won the rest of the 40, which included then-newbies New Mexico and Arizona.
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