I think it's Starbuck.
The reaction of D'Anna Biers at the temple of five is revealing. Since we know that her model will be reactivated, it's for sure the reason for the reactivation is for the other models to find out who are the final five models. We know 4, but the Cylons know scrap. The Big surprise will be that the "final" five won't be models that seeks the destruction of mankind, but the unification of both conterparts.
Cylon Leoben does not cease the tell that Starbuck is "exceptional". She knows where Earth is and with that in mind, she's the key to peace.
After all, why not!