I'm putting money on Axton, with all his shield skills, health skills, forbearance, and even grit, plus a turret to draw fire. Salvador has his Brawn Tree of course, and Maya can at least heal well, but I don't think they can take the hits like Axton can.
Also, how well does "tanking" do for a party of four? Is it even noticeable or useful? Or does each person more or less have to take care of themselves?