None of them. Sorry to dispute the very question but they're all awful. I think it's a massive shame there's only 3 candidates (one of whom is never going to win the nomination - O'Malley). The Democratic Party is huge with many talented governors and senators yet the primaries this time around are ridiculously limited in terms of candidates and have far too few debates. This is the party most likely to win the general election and its basically just a coronation for Hillary Clinton. I miss the 2008 primaries when there was a larger array of candidates, even if they just boiled down to Obama and Clinton in the end.
None, they're all bourgeois imperialists who will do nothing but maintain the current state of affairs, they will do nothing for the working class, here or internationally.
"No matter who wins, the working class loses"
Clinton has too many skeletons in the closet, Sanders is a die-hard socialist. That leaves us with simple choice: O'Malley. Is he a good candidate? No. He's just the best of the worst.
Bernie has a very good chance to beat Trump and many other Republicans.
In a national election, Bernie's democratic socialism is nowhere near as relevant as the fact that his gun policy is nuanced, his foreign policy is much closer to the center, and he has a reputation for integrity and honesty.
O'Malley is actually a very fine choice, but Sanders will bring a new perspective that is needed to the White House. The rich and corporations have had their interests represented at the cost of everyone else's for too long.