I think Coburn is probably the best Senator in office today, but I'm not sure he would be the best VP pick for McCain. He doesn't deliver a state (McCain will win Oklahoma either way), he doesn't offer executive experience, and he certainly won't temper his potentially grumpy/cantakerous image, either.
What is more, conservatives who value Coburn's role in the Senate may object to losing him.
Bottom line: McCain could certainly do much, much worse than Coburn. Still, I think Sanford or Romney are wiser picks, at least, electorally.