Ballan won't ride Paris-Roubaix, as he is still recovering from the consequences of the cytomegalovirus.
My bet's on Boonen. He's got the same form as last year - if not even better - and there's considerably less competition with the absence of Ballan, Cancellara, O'Grady and Hushovd's wrist injury. And seeing he has by far the strongest team around him, only bad luck can keep him from his third win imo. I think the biggest contenders will be Devolder, Flecha, Hincapie, Hoste and perhaps Pozzato if he really puts his mind to it.