Gingrich had the best chance of winning the general last time. But with Santorum siphoning off the whole evangelical wing, they paved the way for Romney.
I personally believe Cruz has the best chance of winning the general. Trump’s gaffes and 30 years worth of a controversial public record will be able to be weaved into a lot of negative ads. There’s already a movie, You’ve Got Trumped, highlighting the same kind of unpopular business record that Romney got hit with in King of Bain.
That is how they planned to get the nomination for Jeb this time but thanks to Trump it hasn't worked out that way.
But all of Trump's potential negatives are taken out when you consider:
1) He fights, which is how he's survived the McCain comments, the invented "sexist" comments, the invented "rape" accusations.
2) Hillary Clinton will be the nominee (the fix is already in for the Democratic side) and will have far worse negatives.
As for Ted Cruz, Cruz can easily be attacked by just running video of his father, Rafael Cruz, preaching before Dominionist crowds detailing how the Christian church is split into two groups: the Kings and Priests, and it's the Kings' job to gain dominion over society and funnel all the money to the Priests. After that, they can then hit Cruz for his obfuscations on immigration-- which most leftists are already familiar with, even if Conservatives aren't. And finally, there's just Cruz's own unlikability after that. Cruz comes off fake, rehearsed. This is how he comes off to normal people.
Cruz will lose the general by a great deal.
Cruz will not win any cross over votes.