Who is the best challenger to President Barack Obama in 2012?

* Tim Pawlenty * Rudy Giuliani * Sarah Palin * Mitt Romney * Mike Pence * Mike Huckabee * John Kyl * Newt Gingrich * Eric Cantor * Haley Barbour * Jon Thune * Jeb Bush * Ron Paul * Rick Santorum * Marco Rubio * other

Who is the best challenger to President Barack Obama in 2012?