It depends what you mean by "doing best." Do you mean winning the crowd over, or do you mean winning on substantive measures? If you mean the former, probably Mike Huckabee. He is a genuinely likeable guy who speaks softly and generally has sensible viewpoints (even when I disagree with them). If you mean the latter, probably Mitt Romney. He is by far the smartest of the main Republican contenders. Ron Paul is a scary extremist, and Sarah Palin is flat-out stupid.
If we're including other potential dark-horse contenders...Mitch Daniels and Chris Christie know their stuff and would probably do well in a debate. John Boehner and Jim DeMint are idiots and would get pulverized.