If I could change the outcome of one presidential election it would be 2000. Not 2016, not 1968.
I think I'd go with 1980. That would most likely butterfly away the Bush and Trump presidencies, stop the Republican "revolution" in its tracks, and it was most heartbreaking because Carter reportedly literally cried when he realized how badly he lost. I will never forgive America for making Jimmy Carter cry.
1968 is also up there because Nixon basically committed treason to win it, and butterflying away Watergate would do a lot of good for the country.
