I think the best candidate is the person who would be the best president. Biden would not have the learning curve of any of the others, and would have the advantage of Long-term relationships that he could build on for the repair work ahead.
And let’s not forget, he is likely the only person who would cause a collective sigh of relief around the world. That matters to me.
He has made mistakes in the past, and occasionally says stuff that he backtracks on, but I don’t think he would make serious mistakes. I also think there are enough republicans who would be happy to get trump’s stench off of them that he just might get some cooperation. On the other hand he wouldn’t need to learn (as Obama did) that you can’t count on GOP cooperation.
Put Warren beside him and I think we would have a winning ticket.
Just my fifty cents...