Edwards would probably be the frontrunner, but I can't see him getting past the convention due to his affair. His best case scenario would be that he wins the primaries, but gets dropped at the DNC, with many of his delegates from earlier in the primary season turning on him.
Could also see Feingold taking up Obama's mantle as the 'progressive'/'change' candidate, and winning against Edwards before then.
Sebelius is also plausible. The others have various issues — Daschle is too much of an 'insider' for the environment of the 2008 Dem primaries, Shaheen would have been out of office for six years and not have served in the Senate yet, and Bayh just lacks any appeal compared to the other candidates, being the OTL-2008 Biden or Dodd of this field.
I thought Jeanne Shaheen was already a Senator