In order of strength:
Daniels, Pawlenty, Thune, Romney, Huckabee, Palin, Barbour, Paul.
I would have had Daniels as a potential front-runner, but I don't think he really wants to run. I also think his ethnic background may hurt him, or voters may choose to ignore it.
Dude ... we have a black President named Barack Hussein Obama who was raised as a Muslim and has referred to his mother's atheism as "healthy skepticism". Mitch Daniels is not going to be hurt by the fact that his grandparents were Syrian Christians.
Hold on a second. Last I checked, Obama was raised by his mother, and later after that, by his grandparents, all of whom were Christian, I believe. And even his father did not exactly practice Islam. Therefore, he was not raised as a Muslim. Obama himself explains it in this quote: