Colin would have been my pick, but after leaving the Bush Admin, I think he has a bad taste in his mouth for Repubs for a while.
Sadly, I feel his chance has passed. He's only 7 months younger than McCain, but by the time the next election rolls around, he'll be considered by many to be too old to shepherd the nation (as will McCain). And that's too bad, because I really like Powell.
Powell (Hearts) Obama Talk about Cabinet potential!
As I said, Colin has a bad taste left over from Bush.