undefeated Iowa State in the CCG. Winner gets the top-4 seed and the loser likely a lock for the CFP, with a strong chance for a round 1 home game.
The next best scenario *for the Big 12* would be 11-1 Kansas State or 11-1 Iowa State beating undefeated BYU in a close CCG while Iowa State or Kansas State misses the CCG with an 11-1 record. That would give the Big 12 two 12-1 teams and at least one 11-1 team. CCG winner gets the top-4 seed and then at least two other B12 teams with sexy 1-loss records for the CFP beauty pageant.
Texas Tech, West Virginia, and Colorado are already tainted with OOC losses...so, a loss to them in the CCG would hurt BYU, Iowa State, or Kansas State more than just one highly-ranked B12 team beating another highly-ranked B12 in the CCG.