My own personal priority is who will deliver the biggest win.
I think for a candidate to do well, he has to be charismatic enough and enough of a blank slate, so that folks put what they want in the blanks, instead of what they don't want. The latter is why Hillary lost- people just didn't like her, so they took the worst of what they knew about her.
The worst Democrat is better than Trump, so winning is the primary goal.
The second criteria I'm looking for is ruthlessness. I want a president who will bend and break whatever rules are necessary to stop the Republicans. They're that much of a threat in my eyes that extreme measures are needed, and have proven that we don't do them first, they will. Ruthlessness only works if you win though, and have enough votes in Congress to enforce your will.
Ideology is my third criteria, but not that important. Ultimately you can only pass what goes through Congress, and there's a ton of low hanging fruit so you can do good just cancelling Trump's executive orders.
The candidates that I think have the best chance of a big win are Harris, Buttgieg, and especially Beto. None of those three have too much experience, and the last two have a real charisma. Harris I'm a little more uncertain about, but I think she could unify the party better than any other candidate.
In terms of ruthlessness: Beto has the edge here, as he's on record as supporting court-stacking. Harris is open to it, but I'm less convinced she'd do it. Buttgieg hasn't gone on record, and has a history of bothsidesing it some, so I am somewhat wary on him.
Ideologically, I'd prefer Warren, but she just isn't likable. Bernie is almost as good ideologically, but I think too many folks will assume the worst with him, and I have a hard time seeing him getting any of the other bases on board.