Not that it counts for anything, given I'm ineligible to vote in this election (no taxation without representation my fluffy arse), but of the current crop, my pick would likely be Warren - she's got a decent track record, and has been willing to listen to the likes of AOC. Maybe Sanders as Veep?
Biden - if we ignore his gropiness, and I'm not especially inclined to, my sense is he might poll decently enough, but wouldn't be able to actually get the vote out; he's a very safe candidate, and simply being Not Trump isn't enough.
As regards polls: let's not forget Clinton did win the popular vote - but, that's not what counts, as we all know. Gerrymandered districts often weigh against the Dems, and the ongoing Rep attack on voting rights does work (for them), even if we discount the lack of audit trails with the electronic voting systems in use, and their demonstrated vulnerability to fraud. Even with a magic candidate, if someone's vote doesn't count, or is outright flipped, there's a headwind against them. Ultimately, what will make the difference is getting people out to vote - ensuring they're registered, and with any proof of identity required.