Harris. Easy answer. Already has name recognition, already has been planning to take over as President (if even a single Democratic advisor isn't brain-dead), and scares the Republicans because she's a woman and black. Sure, she's a cop, but there aren't any good options in the Democratic Party, and she won't put up with Trump's shit.
And isn't past retirement age. Of course she wouldn't have universal support, but I doubt there's any Democrat who could have less support than Biden right now.
As for all the things Biden has done, I have heard of a few trivial steps in the right direction his administration has done. That's great. Why isn't he focusing on them in his campaign? Ignore the Toupee. Tell us what empty promises his administration is going to make. Nearly every sentient being is a better option than the Toupee, that's not a compelling reason to vote for Biden, that's just a reason to vote for anything else.