Which unbeaten team has the best credentials?
There will be no way Auburn, Boise State, Oregon and TCU will be able to settle things on the field if all four teams remain unbeaten; the BCS math just won't work. And if that's the case, it'll be up to fans to determine for themselves whether the best team really won.
Of the four remaining unbeaten teams, which has the best offense? What about the best defense? And if there's no scientific way to settle this anyway, which team do you most enjoy watching?