The only honest answer is no one knows. But if I'm pontificating:
I would say Kamala Harris.
I think there's a pattern in recent presidential races that steers me away from saying Biden or Sherrod Brown. Most people tout those names because they're candidates who could relate to and steal away Trump voters. They're considered "safe". But the pattern in presidential politics seems to be that if you want to win, you should run a candidate who's the polar opposite of the person you're running against. For instance:
-the backlash against ethically dubious, playboy Bill Clinton was to go for the folksy, "family values" touting George W Bush.
-the backlash against bumbling, dumb as rocks George W was to elect someone smart, oratorically gifted and charismatic. Not to mention W totally fumbled Katrina, leading to Kanye's "George Bush doesn't care about black people" statement that I'm sure planted a seed in some minds that maybe we should stop letting mediocre white dudes steer the ship.
-the backlash against cool, calm and collected Obama was to elect someone brash, emotional, easily angered and who weaponized their whiteness as a direct response to Obama being black.
So when I look at who will win in 2020, I don't take the approach of "who is most like Trump while not being Trump" but rather of who is the furthest removed from Trump? Who is the equal but opposite reaction to him, so to speak? I think that is probably Kamala Harris.