As much as many wish to resist the obvious answer to this question, it is Biden. Josh Marshall has a good piece on TPM pointing this out.
"I want to separate here what you may think of Joe Biden and his candidacy and fairly extensive data we have on his relative strength vs Trump. There are a lot of people out there insisting that Biden will be a general election trainwreck for the Democrats...But you simply cannot make this claim about Biden being a weak general election candidate without grappling with the fact that basically every poll for months shows that he is significantly stronger than every other Democrat up against Trump....
To state the obvious, none of this means Democrats have to support Biden. Even this relatively negative poll shows the others very much in contention. But wishful thinking won’t change the fact that the evidence we have to date shows Biden, whatever his faults, is the strongest challenger."
Note that this poll, despite the wide lead for Biden, is actually pretty close to his average lead over Trump (9 points in the RCP running average). Harris, Warren and Sanders are bit low relative to their averages, but their pattern of underperforming Biden has been consistent.
Why is this? Why does run so strongly against Trump. Well, the Post has not released the crosstabs from this poll but the general pattern from other polls has been clear: it's those pesky white noncollege voters who are much more willing to vote for Biden than for the other candidates. And it is those voters, of course, who are likely to decide the Democratic candidate's fate in the vital Rustbelt states of MIchigan, Pennsylvania and Wisconsin.
A final word on "electability". A common response to these kind of data is to entirely deny the relevance of the electability concept to current candidate assessment. It's too hard to predict accurately who will run well in a general election context so we shouldn't try. But that defaults to an implicit assumption that all candidates are equally electable, which makes no sense. Voters will and should make a judgement about electability and they will and should available data to do so. Ruling the question out of order is a cop-out.
A related claim is that candidates may not be equally electable now but once nominated they will become so as voters come to see them as viable. No doubt there can be a validation effect here but to assume that will overcome all apparent differences in electoral appeal strikes me as delusional.
Like it or not, electability counts and right now Biden appears to have more of it than the other candidates.