Keeping in mind that Biden hasn't been a senator since 2009, during his time in the senate he was dead center among senate Democrats in terms of liberal/conservative votes cast (i.e. both more liberal and more conservative than 50% of his fellow Democratic senators): https://voteview.com/person/14101/joseph-robinette-biden-jr . Of course as noted this was over a decade ago, and what passed for centrist back then might not today.

In case you're wondering how this metric pans out for the most recent Senate: https://voteview.com/congress/senate/115/text , which (according to this metric) lists the four most liberal senators (as shown by their senate votes) as, in order, Warren, Harris, Booker, and Sanders.