Biden is really messed up. Seems like dementia. He has major trouble with putting together coherent thoughts. I think if Hillary or Michelle Obama were nominated, they would get slaughtered. Middle of the road voters would stick with Trump.
Honestly, I think Bloomberg would have the best chance, but the progressive wing won't support him. In the debates, he will say something really smart then in the next sentence completely undermine it with pandering to some "marginalized" group. Plus, the Democrats won't see him as being different enough from Trump. They hate billionaires, unless they're getting money from them. Then they hate them a little less.