The Perfect Running mate for Mitt Romney would either be a Hispanic or a woman. These are demographics that are untapped by Obama's Administration. Also, the running mate would have to have plenty of political experience and be from a vital state. Plenty of experience shows that they know what they are doing and if the state they are from has a good amount of electoral votes, it is even better. With Romney being a "moderate conservative" you really have to keep the interest of the "extreme conservatives" which is the Tea Party. These are important votes that normally always vote Republican, that cannot be lost.

Who is the perfect candidate for Romney to run with that matches all of these categories?
Marco Rubio. A man with Latino roots and someone who would actually be able to give their acceptance speech in Spanish, would give great interest for Latinos and could very well win them some important votes. Currently he is the Speaker (and member) of the Florida House of Representatives. This is good experience for later on. His good ties with the Tea Party just practically guarantees votes that might not have been won since he is seen as a "conservative" Republican. These are a lot of positives that would go in favor of Romney by choosing Marco Rubio.

Marco Rubio

The worst choice for Romney to run with would have to be John Thune.
Thune is a 50+ year old white male, which demographic majorly votes Republican as it is. Romney also fits into that same category which would seem redundant to have Thune as his running mate. He is considered "safe," he doesn't anger anyone really but does not really get anyone excited either which would hurt the Romney campaign. This seems like a useless choice to me and there are a lot better of choices that could be made to help the Romney Campaign.

John Thune