There are four major players in the primaries now: Clark, Dean, Edwards, and Kerry. Of our two northern liberals (Dean and Kerry), Dean has the best chances of defeating Bush going by his resume. Whether he can take the lead again is now the million-dollar question, but if Kerry wins the nomination he will be beaten worse than Gore.
Clark and Edwards will do well in SC and OK. Clark may do even better in AZ, NM, and ND. Edwards, I think, has the best chances of defeating Bush overall because of the southern swing states.
In closing, unless Dean can beat Kerry and appeal to southern voters, an Edwards/Clark (or Clark/Edwards) ticket is our best chance of defeating Bush.