Right now...honestly...I think it's Gov. Sarah Palin.
If you Republicans want to win back the White House in 2012, I can guarantee to you now in 2008, that Sarah Palin won't be your knight in shining armour that will lead you back the good old days of Reagan and Co. People, a part from the Religious Right wing of the Republican Party will release that she isn't electable on the national scene.
McCainforPrez is right, Palin is the driving force behind the GOP right now. But I think "head" is a poor choice of verbage (Palin joke, huck huck huck) since I don't think she makes any decisions yet and won't for a couple years.
I'm extremely confident that she's heavily favored in the 2012 GOP primary should Obama win - she'd have four years to get more competent at interviews/debates, she'd be fully vetted
, she'd have massive fundraising appeal & connections, and she'd generate enough enthusiasm to dominate Iowa like Huck did. She'd be like Huck but with Mitt's money and Hillary's establishment support (but for Reps).