For whatever it is worth, Joe Biden and Bernie Sanders are the two best running by the metric of “polls well against Trump and actually has an electoral college path to winning.” Pete Buttigieg and Elizabeth Warren, I think, still poll admirably against Trump overall but their swing state polling that will decide the EC has always seemed much worse than Biden or Sanders.
Bloomberg‘s big problem is that almost every quadrant of the so-called political compass hates him for some reason or another. He doesn’t seem like a honestly bad guy. Centrists try to scare left-leaning voters into thinking Sanders would be a Mondale type candidate but I actually think Bloomberg would be that type of candidate in 2020. Right leaning people see him as a part of the liberal nanny state who will take away their coca colas and cigarettes. Left leaning people see him as a part of oligarchical class that will throw them to the wolves for billionaire special interests.