Bloomberg wouldn't be my first pick, but if he managed to win the primary he'd have my backing. The ultimate goal for all good Americans is to vote Trump out in 2020. So long as his successor isn't somehow worse (all the Democrat candidates are exponentially better), they'll be an improvement.
On that note: while I don't want to take anything for granted, I think we have a habit of underestimating the likely turnout of Democrat voters in November. It's easy to worry about the vote when there's been a lot of bickering between Dem candidates, but when people realize that the Dem candidate is the only hope of restoring sanity, I don't think they'll particularly care who the primary winner is. The important bit is ensuring that enough people in battleground states realize that.