Warren or Harris, IMO. It's still early days, and people are expecting a Biden entry which may or may not happen. They're....biding their time, you might say.
But seriously. Warren would undoubtedly make a better President, but she will have weaknesses as a candidate (see her DNA test nonsense, for example). I think the converse is true of Harris, and if Biden doesn't enter, the centrist Dems will rally around her. It's a matter of style, as much as anything...they're both lawyers, but Warren was academic, while Harris was a public prosecutor. The latter know how to go for the throat, play the crowd, make compelling arguments and tear their opponent down in a way most of the former simply don't, as a matter of practical experience. Once the debates start, she will be in her element.
As for Sanders....I think there is a lot of bad blood there, and there are former Sanders supporters among those who think he shouldn't have run again. Warren gives them a similar candidate without the same baggage, which relegates Sanders to Ron Paul status - he'll be popular online and in the online polls, but he'll struggle elsewhere.
Also look out for Andrew Yang siphoning off some of the more weird Sanders support base.