For me it's a close tie between Gaara and Ohnooki. Gaara defeated the previous kazekage (his father) and the 2nd mizukage (with some help from Ohnooki), and provided balanced offensive and defensive support against Madara.
Ohnooki probably did the best against Madara overall and stopped his predecessor (at least he thought he did), but really only with Gaara's help. He did help Gaara against the Mizukage, but he might not have survived the battle without Gaara.
Raikage, sorry...you were disappointing. You defeated no one.
Tsunade, yea...you saved everyone from dying at the end of the battle with Madara, but that's really it.
Mei, were you even fighting?
Ohnooki probably did the best against Madara overall and stopped his predecessor (at least he thought he did), but really only with Gaara's help. He did help Gaara against the Mizukage, but he might not have survived the battle without Gaara.
Raikage, sorry...you were disappointing. You defeated no one.
Tsunade, yea...you saved everyone from dying at the end of the battle with Madara, but that's really it.
Mei, were you even fighting?