Democrats for whom I would very happily vote against Trump:
Democrats who would not be my first choice but for whom I would still vote against Trump:
Democrats about whom I have more serious reservations but for whom I would still vote without a second thought against Trump:
Bill de Blasio
Democrats I honestly couldn't pick out of a lineup but for whom I would still vote against Trump:
Seth (?) Moulton
Democrats I actively distrust but for whom I would still vote against Trump:
Non-Democrats for whom I would vote if I had to against Trump:
Any Bush who decides to run
I have some policy complaints about Biden and more serious worries about the way he campaigns, the reasons he's been unsuccessful before when running for president, the fact that he's not great at debating and in fact I hope he'll refuse to do the media-staged TV debates because while Trump's voters largely don't care if he tells outright lies and sounds like English is his second language, Democrats have increasingly shown how readily they'll eat their own. My policy complaints about Biden are overwhelmingly on environmental and health care issues. On paper, I agree with Sanders on some of those issues more than I agree with Biden, but Sanders can't even put together a group of his fellow legislators to tackle those things, let alone a broad coalition at state and local levels as well as nationally. I also can't stand many of the people he has around him, several of whom I would expect to be in his administration. If I'm choosing between Biden and Sanders, there is absolutely no contest: I trust Biden more as an executive, I trust him more as a human being. I have greater confidence that he can beat Trump because he will bring the most Democratic voters to the polls.
It's more complicated for me when it comes to Elizabeth Warren, whom I actually really like, though I acknowledge that the places where she's most problematic involve racial inequality and it's a reflection of my privilege that I like her in spite of them. There are specific reasons that Warren was the first choice of less than 5% of black voters and why most of them trust Biden much more than Sanders. Except among voters too young to fully understand why the Cuba comments are so controversial, this is true of Latino voters as well, and black and Latino voters are now the core of the Democratic Party, not white women, who last time went for Trump in higher numbers than they went for Clinton. I have really disliked the implication that anyone choosing Biden over Warren must be doing it out of latent sexism rather than focus on specific issues, and I have really resented all the "Smart voters vote Warren!" posts of the past week, which imply that the vast majority of voters of color, who won't vote Warren, are less intelligent or less thoughtful. Black voters were Hillary Clinton's strongest supporters at the polls, so they did not reject Warren (or Klobuchar, or Harris) purely out of sexism.
Are women held to a higher standard than old white men? Certainly, but the old white men in this race have had decades to prove their bona fides -- or, in the case of Buttigieg, only a few years, which underscores the reason I think he needs more experience and more time to prove his values with direct community engagement before he'd be my choice for the top executive position in the nation. Most black voters have made clear that they trust Biden's work over the past decade more than they worry about stupid things he did in the past. The same does not appear to be true for Sanders, whose civil rights work peaked decades ago and who often sounds oblivious to issues of specific concern to black voters and women. As for Warren (and Klobuchar), they have never made civil rights issues a top priority, and the assumption that economic policy will fix racial injustice has proven time and again to be false, whether the policy is more centrist or more progressive.
As I said, the point on which I agree with Sanders most strongly is that we desperately need better environmental policy. This is why it's so important to vote for Democrats in the House and Senate and in our local elections, not just the White House. However much anyone may like Sanders' policy on paper, he will need a coalition of legislators to push it through, and I've seen no evidence when studying his long career that he's capable of or even interested in putting that together, only of getting people to heckle non-supporters. I will vote for him if I have to, if it's the only way I can stop Trump from appointing one more Supreme Court justice which will spell the end of democracy as we've known it, but let me tell you, I will be crying.