Which Republican would be the toughest opponent in 2016?

This is putting aside the question of who has the best chance to win the nomination, and certainly putting aside any question of who might govern the country slightly less badly than the others. I ask only about his chances in the general election, assuming he gets the nomination.

The DU software can't accommodate the full range of the Republican clown car. I winnowed by using
Chris Cilizza's list in the Washington Post of the ten candidates with the best chance of being the Republican candidate in 2016, except that I had to drop his #10 (Mike Pence) to make room for the "someone else" option.

As a side note, not even making Cilizza's top ten are the retread candidacies of Rick Perry and Rick Santorum, the novelty candidacy of Ben Carson, or the "I hear a call that no one else knows is there" candidacies of Carly Fiorina, Lindsay Graham, and Peter King.

Cilizza put the candidates in order of likelihood of being the nominee but I've alphabetized them.

So, who's their best shot at getting to 270?