According to VoteView, Boxer, Kennedy, Corzine and Sanders had the most liberal voting records during the years when Obama and Biden were in the Senate. Biden was in the middle of the Democratic pack. Obama was a little more liberal.
I thought I had read somewhere that Obama had the most liberal voting record and Biden was second, but maybe that source was incorrect. Different groups take different votes to evaluate politicians by so it could be that different organizations rate Biden further to the left and others more to the center of the Democratic pack...which of course is almost completely left of center.