Well lets see based on the ones in the poll
Huntsman ---> By far my favorite candidate but gives nothing to the ticket, since he's viewed in a similar fashion to Mitt. Would be useless as the VP pick. Bad choice
Santorum ---> Would help shore up some of the evangelical base, but will drive away independents and turns an election that needs to be economic focused into a partially social one. Bad choice.
Cain ---> Not very experienced, but as VP that's less of an issue. Popular with the base which would help shore up a location Romney's hurting at. Private sector experience and economic focused which could help this election cycle. Could actually be the best choice out of this group.
Perry ---> Solid with the base, and as VP his debate skills won't be as big of a deal. However, such a large personality that it may not work as VP. Not a bad choce, not a great one.
Bachmann ---> See Santorum.
Gringrich ---> Would help a bit with the base, would trounce Biden in the little bit of VP debates that happens, and would be a good fit as VP. However, his baggage may hamper him way to much. Similar to Perry, not bad but not great.
Of the picks you've put forward, I would say Cain would be the best bet for Romney to pick as VP