Probably Huckabee, but 2008 seemed loaded at the time. Anyone to stop Romney except perhaps Fred Thompson or Hillary unless they were a last line of defense.
I would have enthusiastically voted for any of the following:
1. Huckabee
2. Edwards
3. McCain
4. Paul
5. Obama
All of them have seriously declined since that time sadly. If this were today, I guess Paul perhaps?
Um, Huckabee and Edwards were polar opposites of each other...especially in 2008, with Edwards running on his pro-Daily Kos line of thought.