i won't respond to your question, because it is outside of the realm of reality upon which elections are decided.
general elections have very few debates, and MANY news cycles. the debates matter, but winning each news cycle is what matters now. toomey was very effective in implementing this strategy in 2010 and won over a late-surging sestak. obama mastered this in 2008. the debates were judged to be about even, but obama won many more news cycles. this is why your question is a distraction, not a sincere inquiry.
however, if i were to answer your question hypothetically, i would answer like this: in a debate, romney fails hard. he appears to be automaton. gingrich would be the best in a debate, but remember that he can only call obama a "saul alinsky radical" or "food stamp president" in a primary, not a general. in a general, that shit would be eaten alive by obama's machine. the fact that he does not bother with gingrich shows how little of a threat he is. i am laughing as i watch republicans fall for him...ALL OVER AGAIN! santorum draws a distinction, and is a real threat upon examination. he appeals to blue collars and seems to have mastered an "aw schucks" look lately. if i were a hypothetical republican looking for someone to defeat obama, santorum would appeal to me highly.
so, in a debate: gingrich/santorum, romney leagues behind.
to me, "mano a mano" is the news cycle of the equation. romney wins there, but has major setbacks such as a history of really odd gaffes. gingrich is a close second, he is sharp but he plays checkers, not chess. santorum is the slow but steady horse that would lose with some class.
basically, you guys need santorum's blue collar appeal, newt's sharpness, and romney's rich guy douche bag appeal. but you only get to choose one of them.
sucks to be a republican if you are one.
