I'd be stunned if it wasn't Jim Prentice. From the point of view of someone who wants them to lose the next election, that's worrisome, since he's actually kind of likeable.
Kenney is a dark horse, though. He's got a big base of support among immigrant communities, apparently, and if you can win over some of those blocs (i.e. Sikhs in BC), they can deliver lots of votes.
Mackay stands no chance...he's kind of dim, and he doesn't have any power base.
Baird...heh.
Not sure who else they have, since Harper's destroyed most of his would-be successors...Tony Clement? James Moore? Maybe Maxime Bernier really will make a comeback?