I doubt Michelle Obama, but I'll be very surprised if a woman isn't the Democrat nominee in 2020.
Even though she's now a two timer loser and people of all political stripes seem to be tired of her, Hillary is like a zombie that won't stay dead. So she could stagger back into the ring again. I wouldn't be entirely shocked if she did. Although they probably don't think so, both Kamala Harris and Kirsten Gillibrand are still regional political lightweights and don't have enough name recognition, IMO. Before her (once hidden) role in the hijinks and scandal at Facebook started coming to light, I would have said Sheryl Sandberg. She's a major force in the U.S. and global feminist movement with LeanIn, she's rich, Jewish and politically connected in Silicon Valley and the State Dept. She may be damaged goods now though - especially if Zuckerberg puts a dagger in her back to save himself. Course, there's Elizabeth Warren. I think she has a pretty good chance of winning the nomination. She's better connected to the dominant "progressive" faction of the party than any other potential major candidate that I can think of. As for what chance she'd have against Trump, it's hard to say.
As to who would have the best chance against Trump... I don't know. Like always, it's going to mainly depend on where the economy is in 2020. If the tariff spat hasn't been solved and/or the Fed keeps jacking up rates, I think there's a better than even chance that we'll slip into recession by then - nothing devastating, but I could see economic growth being hamstrung enough for that. We're overdue for it anyway.