I would argue that Biden is the most likely. While Sanders, Warren, and Harris show that the extremist wing of the party is getting stronger, at the end of the day most Democrats know that the objective is to beat Trump, and that can only be accomplished by convincing swing voters in swing states to vote D. And those swing voters showed in 2016 they aren't going to automatically vote against Trump. So you better not scare them off with an extremist.
So the D's need a moderate, and Biden is the only famous moderate in the race. The other moderates are people that the public doesn't really know. So as much as Biden is an imperfect candidate, he's the one that maximizes the D's chance to win in 2020...and that's what matters, not a protest candidate trying to win an auction of how much free stuff they can promise people that everyone knows they can't deliver.