I REALLY like Huckabee, twin.
At the same time, for political purposes, I am not sure how much he would bring to the table. He is certainly unlikely to attract most disaffected conservatives. In particular, he has had some of the same perceived "weakness" on the immigration issue that McCain has had. He is also from a state, and a region, that McCain is likely to win anyway. On the positive side, he and McCain would get along well and make a good team, and Huckabee is probably the best speaker and thinks better on his feet than any other candidate in either party - to include even Obama.
Romney would be another good choice in theory at least: conservative, strong business and economics background, and from the Northeast. The drawback would be that there is a risk Romney would not "click" with the voting public any more than he did in the primaries. There may also be the personal chemistry problem.
Generally VP selections do not help a candidate much, but they can cause heavy and even fatal damage. Politically, the first rule is - do no harm.
MWL