I'll start:
Mitt Romney - Jeb Bush - establishment favorite, semi-moderate record, not well liked by the grassroots supporters.
Newt Gingrich - Donald Trump - wacky proposal ideals, considered the conservative alternate but not well liked either.
Ron Paul - Rand Paul - Isn't this obvious?
Rick Santorum - Ted Cruz - very conservative Christian type who may be the fall back guy in case Trump falls like Santorum was when Perry and Gingrich fell.
Rick Perry - Ben Carson?
Michelle Bachmann -
Herman Cain - Ben Carson again?