Bob Graham, as he would have greatly enhanced Gore's chances in EV-rich Florida.
Gore-Gephardt.
Gephardt would have kept some progressives from voting Nader, especially with NAFTA.
They would have kept TN and MO, they would not have needed Florida.
Graham was not someone who would be president in 2008....he was aging
Gore-Gephardt 289 EV
Bush-Cheney 249 EV
I don't think the VP effect would have worked unless Gore showed some willingness to overturn NAFTA. To use an example, Pence only worked as a VP because Trump showed a willingness to advance the ideas that Pence represented (i.e. Christian conservatism).