The more I think about it, the stronger chance I think O'Malley has. His speaking skills are good enough, he has executive experience, excellent fundraising and campaigning skills, no cluster-effs like in New York (where Clinton, Cuomo, and others would all draw from the same local sources), he doesn't have a potentially controversial voting record in the Senate (which, with the Senate's low approval ratings and stuff like the sequester, healthcare, unpopular deficit reduction measures, etc., is a huge boost), he hasn't done anything outrageously bad as Mayor or as Governor, and he successfully held his seat in the 2010 Republican wave (I'm always skeptical of one-termers--that could be luck, whereas winning multiple statewide elections means you have skill).
If not Biden or Clinton, I think O'Malley is a really strong bet.
He's photogenic too:
