All this discussion about Biden's possible VPs got me thinking back to all the elections I was in America for. So, let's analyze what happened in those. Maybe, we can figure out whom Joe needs to pick.
Let's take the wayback machine all the way to the heady days of H.W., then...

1992: Clinton picks Gore, ignoring geography, choosing the political balance instead (Gore was strong on the environment, while Clinton wasn't). Not a bad pick overall, but certainly didn't decide the election. Bush's broken promises, recession and Perot did, in this order...

1996: Dole picked Kemp for utterly mysterious reasons. Geography never came into play, but maybe Dole wanted to shore up the right flank (he was a moderate, the culture war was raging). Didn't help and couldn't, really, as moderates liked Bill better than Bob...

2000: Gore picked Lieberman because he wanted to run as far as possible from Clinton's sex life. W picked Cheney because W was a moron and needed someone who knew Washington to run the country for him. Didn't really matter: goobers voted for Bush due to his "likeability", anyway.

2004: Kerry picked Edwards because he knew he came across as an elitist and needed a good ole Southern boy to appeal to the working class. Never worked, even for a minute, as the Bible thumpers came out in force to vote against gay marriage...

2008: Obama picked Biden because Obama knew next to nothing about foreign policy. An amazing pick from all angles: he got a great VP and a friend, and it's likely Joe helped win over working-class types. McCain picked Palin to get the goobers into a frenzy...

McCain's plan worked spectacularly to that end, but utterly alienated the center. So yes, 2008 was one election where VPs actually had an impact. But not nearly as big an impact as the Wall Street meltdown...

2012: Romney picked Ryan to convince the base he was a nutty right-winger, too. Didn't work at all, because Romney's problem was being a cold-hearted bastard with zero empathy. Turns out, picking another one of those didn't do much to address the issue...

2016: Clinton picked Kane because she arrogantly believed she had the Obama coalition in her pocket and needed to mollify white men. She did win Virginia, narrowly, but white men were super not into her. No VP would've helped that...

Trump picked Pence because he needed to convince evangelicals that a thrice-married pussy grabber is the answer to their prayers. Utterly unnecessary, since evangelicals are a bunch of phonies who would vote for a child molester if he ran as a Republican, so Trump won them anyway

Trump's key constituency were blue-collar whites who love him because he is an asshole, like them. VP picks were utterly ineffectual for both parties...
So, what's the conclusion?

It's pretty clear VP picks generally don't mean a whole lot, but when they do, their impact is generally limited geographically or demographically. Meaning, one particular state or one particular demographic.
So, Joe would need someone who can bring in one state AND one key demo.

With that in mind, to me, the obvious pick is Stacey Abrams.
While Whitmer can bring in a key state and Warren can shore up the Bernie flank and Harris might have an impact with POC, Abrams both makes the Dems competitive in a big state (w/ 2 Senate races) and helps with a demo.

Energizing the black vote is, in my opinion, more important for Biden than appealing to blue-collar whites. The percentage of non-college whites who would switch to him is negligible. White suburban women are already his. Big turnouts in cities is where it's at for him.
So there.