Two horse race Kessler and Calzaghe. The likes of Andrade and Mundine MIGHT be able to pull an upset if everything goes their way. But it seems unlikely.
Right now - as in who'd win if they fought right now and past accomplishments don't mean shit. Then I'd have Kessler on top by a small margin.
He's had the best recent oposition and looked the best against it. Also Kessler being far more willing to take a Calzaghe fight, than the other way around indicates perhaps Joe is not at all confident.

If Calzaghe can pull of another perfect performance like against Lacy he sure looks dangerous. But it'd better be a career best performance because Kessler fights very consistantly on a high level and has not revealed many cracks in the armor.
Much more uncertain what state Calzaghe would show up in for the fight, with age and hand-trouble possibly beginning to be major problems.

As time goes by, the odds are increasingly tipping in Kessler's favour.