Not for the faint of heart. Olivares was historically greater, but I think I'd take Zarate to win this one. The guy was a killing machine before he moved up and ran into Gomez.
Olivares KO7 ... I think he's able to consistently get his left hook home and make Zarate miss ... besides, if that fails he can back off and box him ... just think he was better and more versatile and would win If it's Olivares after he beat Castillo for the third time, then it's a wild card because he'll either train and be good or get hammered and fat and then show up a weak, weight drained shell and get killed ... but I assume both are at peak