If I was in a swing state like Ohio, I'd probably force myself to vote for Romney -- even if his running mate were an uber-RINO like Olmypia Snowe -- because the disaster-in-chief MUST be defeated and I can't think of anyone who would make a worst President. Hillary would be an improvement over Obama, ugh.
Of course, I'm in "Obama's home state" (sadly) and he's not going to lose Illinois unless it's a McGovern-style disaster for him on election day. Since my vote doesn't matter, I won't waste on Romney if he picks a running mate that annoys me. Fortuno the RINO, pretty boy porker from South Dakota, and Rubio after his "I would come here illegally if I had to" have probably crossed that line where I don't trust them (and I don't trust the guy on the top of the ticket either, so I have nothing to work with). I wouldn't repeat what I did with Mark Kirk and vocally campaign for a third party candidate, but when it comes time to vote in the privacy the ballot booth, I can't see myself casting a meaningless vote "for" a Romney/Fortuno ticket.
My two cents...
McDonnell, Pawlenty, or Portman - not necessarily in order of preference or likelihood.
Romney will not pick a dark horse after what happened with McCain (although if he surprises along that line, I hope the dark horse is Gov. Martinez). He’ll also not pick someone who will overshadow him charismatically (not that hard to do), so that rules out Rubio or Ryan.