Edwards.
The Dean vs Kerry battle in the 2004 primaries would have turned into an Edwards vs Dean battle. The only thing that held Edwards back in 2004 was his lack of media attention. No one really paid attention to him until the De Moines Register endorsed his candidacy.
And Edwards, had he picken a good foreign policy expert VP like Clark, would have done very well. He would have paid more attention to economics and brought Ohio at least to the Democratic column though perhaps lost New Hampshire.
Nah, can't go with Clark...an Edwards/Clark ticket has a total of 1 (!) electoral victories under their belts. It'd be Biden or Bob Graham.