Considering the vast nothingness that is Obama's knowledge of anything non-Marx... I'd say any of them would eat his lunch.
I dunno. Rick Santorum for instance has been coming across as rather snarky and angry instead of persuasive and compelling. If Obama has the right debate coach, he could take advantage of that. Ron Paul and Rick Perry are both not that good shifting gears or thinking on their feet if they get blind sided by an unexpected question or comment.
Romney would hold his own but I wonder if he wouldn't continue to come across as if he is reading off a campaign brochure instead of speaking from the heart? Bachmann also sometimes comes across that way, but if she is well prepared, I have no doubt she could hold her own. Also Johnson. I don't know Huntsman well enough to evaluate it.
In the end Gingrich and Cain stand out as smart, astute, well prepared, and able to field anything that is thrown at them.