Now that there is a Mitt Romney 2016 megathread, and for the record, I do not believe for a moment that Mitt will run for President in 2016, or ever again, but that aide, for the purposes of sheer speculation, let us say that Mitt did run in 2016, knowing what we know now, who would be the best running mate for Mitt?
Give your reasons for your choice or choices if you like.
Susana Martinez would be a great running mate for any Republican, including Romney. Other than her, it would really depend on what type of campaign he plans to run: For example, if he wants to go the moderate route then Susan Collins would be a perfect running mate, as she would attract women and centrists, solidify Romney in New Hampshire, and make Maine competitive. Mark Kirk could foreseeably serve a similar purpose, but his leaving the Senate would give Democrats an automatic pickup. If he takes a rightward turn, Rand Paul could help him attract younger and more libertarian voters. That's all I have for now.