I honestly have no idea.. They don't exactly have a strong lineup at the moment. I'd say Bernie but he's getting up there, what is he in his mid 70's now? It's a bummer, as he's about the only Dem I'd consider voting for, along with Dennis Kuscinich (though Dennis is more of a fringe guy and I don't think he'd be a popular choice amongst the mainstream, neolib types)..
I think Warren hurt pretty much any chance she had with the phony Native American heritage fiasco, maybe Joe Biden? Though many might just view him as Obama pt. 2. I could honestly see Hillary having a big enough ego to run again, though the Dems would probably discourage that as she's extremely unpopular. Hell even within her own party she struggled to beat Bernie (a freakin Democratic Socialist) and mostly had to rely on Superdelegates to get by him.
"We hold these truths t-be self-ful evident. All men and women created by the.. Go-you know the.. you know the thing!" - Joe Biden