If you want an opinion, your best shot at the presidency is Biden. He may be classical in his approach, but he caters very well to the independent crowd (something I never see brought up in elections considering their influence) and his message works in the Rust Belt States that Trump took such as Wisconsin, Michigan, and Pennsylvania. As an independent person who is fiscally conservative but mostly libertarian everywhere else, Biden would be the Dem I would most likely vote for. His issues are his age and how he appeals to the millenial generation, but it's one of those things where if you want to overtake Trump you must make compromises.
Bernie is a bit of oddball in that he is older like Biden but his politics just don't work with a majority of Americans. Socialism is still hated by many on the right and in the middle, so you have to hope that his charisma carries him throughout the entirety of the Dem's base, which I don't think it could. He's an older white dude which turns off certain communities in the Dems camp, and he's socialist which will likely disgust many of the slightly left leaners in the camp as well.
As far as Harris or Booker, they would have the minority vote, but they are sorta relegated to where they're from. Harris is from CA, which will turn off many Rust Belters, and Booker really lacks any sort of personality from what I've seen of him. An AOC type to me seems to me unfeasible purely because independents I'm around absolutely despise AOC and people similar to her.
I myself only voted for Trump because Hillary was far worse, but looking at this years nominees, there is absoluetly no one I have seen so far minus Biden that makes me think that they could certainly beat Trump.