1972 with no hindsight, for the same reasons everyone else voted for him that year. Things were going pretty well under him for a while, his policies turned out to be pretty moderate and reasonable for the most part, and McGovern was a weak candidate. Knowing what I know now, of course, it’s out of the question however.
1960 with hindsight. Would have saved JFK’s life, and maybe prevented Nixon from getting as bitter and corrupt as he got. Though who knows how Cuba goes down if Nixon is president then?
1968 I would have voted for Humphrey regardless. Without hindsight, I would not have trusted Nixon or the GOP with their dogwhistling about “law and order” and claims of a top secret plan to end Vietnam. And I still would have preferred LBJ and the Democrats’ domestic agenda. With hindsight, I know Nixon possibly sabotaged peace talks to help his election, so that makes me even less likely to vote for him.
You really don't know much of Nixon's electoral history before that, do you?
Then again, Nixon winning '60 does have the dubious benefit of JFK saved for '64 or '68 depending on whether or not he takes on President Lodge or not, and fewer dirty campaigns down the line.