I'm surprised to see some people here named Kamala Harris as a preference to win.
Harris is a bad egg-- with a history full of overly aggressive prosecution of victimless crimes. Her naked ambition drove her to engage in the most absurd prosecutions just to make a name for herself. For example, the
prosecution of backpage is obviously outrageous even if you view the entire thing through the lense of law enforcement's own claims of what happened.
In her role as prosecutor she abused the states power to the maximum extent possible and treated the rule of law as just a PR game. And she was effective at it. There are other candidates who might aspire to such abuses, but for the most part they haven't demonstrated the competence to pull them off.
With Google manipulating the public in her favour her odds of winning might well be pretty good. But I think she has enough of an unsavoury history that enough on the left will not be hard to convince to stay home to let trump take the reelection. In spite of the sound and fury in the media, many Americans feel that they are better off in recent years at least financially than they were during most of obama's second term-- and it isn't hard to make a case for better the devil you know.
As far as Biden's odds go-- I think there is a lot to be said about the strategic value of him vs trump. But he is impressively old-- several years older than Trump who would himself be the oldest person elected president if he is re-elected. When McCain/Palin ran there were many people who voted against who otherwise would have supported McCain because of the considerable odds of Palin becoming president. I think all the candidates who are older than trump (Biden, Sanders, who else?) will have their presidential odds heavily influenced by who they choose as a running mate.
Interestingly, I believe my views are opposite Theymos' relative to how the odds change with an economic downturn.
I believe that if there is no downturn (or esp with an upswing) then the only candidates that have a chance are ones like Biden who offend few and seem non-threatening to most interests but will pick up the ANYONE BUT TRUMP voting block. If you're happy with how things are, you can trust that someone like Biden is not going to upset the apple cart too much-- maybe even less than trump, as many people still do worry that trump will accidentally escalate a twitter fight into a shooting war (since the media hasn't been tirelessly over-hyping that risk for the past two years I think people are less wary of it than random trump scandal dejure).
If there is a major economic event, however, the status quo will not be what people want, and candidates like Yang or Sanders (and sadly, Warren) would pick up the greatest boost-- much more than a more boring player.
Maybe my position is different from Theymos because I've seen literally none of the recent campaigning or debates? Is that the source of the discrepancy?
This sort of analysis should keep in mind that normally an ordinary recession takes a rather long time for it to really upset the public... at time goes on the downturn probabilities will start to require a catastrophic event in the economy and not just an ordinary recession.