Democrats are most worried about Rubio-Kasich
Rubio-Kasich would be my pick as well, Kasich can’t get traction as a nominee but is a better establishment figure than Bush III or the potentially abrasive and certainly corrupt Christie.
Rubio appeals to conservatives of all stripes. He is telegenic, articulate when he isn’t thirsty and appeals to the third of Latinos that swing Republican. His down side is his lack of experience, and that he appeals to conservatives of all stripes. If he gets the nomination, as the smart set predicts, he will need a moderate adult to balance the ticket more than a even more right-wing candidate or token female/minority (though Condeza Rice could wear all hats, if people could forget Bush).
As an extra bonus, Kasich could deliver a must-win Ohio, while Rubio delivers must win.