For me it's Benson, with Cain/Rousey tied for very close 2nd. Benson went 3-0, beating the champ and defending the title twice (I know that the rematch was close). Cain got his groove back (Tito voice) "with a dominating fashion", as did Rousey.
Mighty Mouse went 2-0-1, but I honestly think that he lost the first McCall fight, which taints his tournament wins thereafter. Everyone else in the list had some fun fights, but get points off for strength of schedule (that is, beating a retired Bonnar or an aging Belfort is commendable, but not as impressive as beating someone in your natural weight class, with both opponents in their prime).