It’s hard to quantify “most conservative”, but, personally, I’d start with the group of governors who declared that they’d reject the stimulus money last year and narrow it from there. That’d be Sanford, Jindal, Palin, Barbour, Riley, and Perry. I might have missed some others, plus Palin is no longer a sitting governor and Sanford would be a non-starter now for obvious reasons.
I don’t think Jindhal or the other Southern guvs have a prayer. Perry strikes me as the most ‘Scott Brownish’ (certainly in looks), though he seems poised for a veep slot rather than the top of the ticket (and is he like Bush on immigration?)
I suppose Mike Rounds from SD doesn’t have a chance?