For the Tories, it's pretty much down to May, Gove, Osborne or Boris.
Out of the lot of them, Osborne's probably the best choice. May is too much on the reactionary right for most of the party hierarchy, Gove doesn't have the presentational abilities and Boris is too much of a gamble. There's a couple of young up-and-comers, but no-one really stand-out. Liam Fox would also stand, but I don't think he'll get very far.
For the Lib Dems, Tim Farron. Danny Alexander is a good outside shout, but there's a fair chance he'll lose his seat. David Laws - normally too divisive - may take it simply by dint of having a very safe seat. I'd have liked to have seen Jeremy Browne go further, but he's standing down. If the party wanted to swing to the left, Vince Cable would be a possibility even despite the fact he's near enough an antique.
Labour? Christ knows. Ed Balls? He's got a more human side, although his association with Miliband and Gordon Brown may count against him.