I would actually have to say Dewey in 1948. His campaign was pretty much sh**t.
I concur with True Democrat. Governor Dewey should have easily encountered for President Truman in '48. I wouldn't be surprised had Taft won the nomination in '48, that he would have done better than Dewey. (Even though one EV map of mine on the Election What Ifs board, is quite the opposite).
I would also have to say Governor Michael S. Dukakis of Massachusetts. Now he should have defeated Vice President Bush in 1988. He had a 17 point advantage over Bush after both Conventions but then it slipped away. We can thank his PR team for that, as HappyWarrior said and also his unresponsiveness to Bush's attacks on Dukakis about Willie Horton.