The Yamato/Musashi's 18" guns were not as effective as they should have been. The shells were optimized to travel well through water. (The theory being that a short would hit the opponent's ship below the armor belt and do more damage). The Iowa's has functional radar fire controls so they could more likely score a hit before the Yamato's optical sights had a solution.
Needless to say this is an old argument. Probably the fight would go to who made the first hit.
Bismark was far inferior to both. Basically a WW1 design, armor optimized for relatively close ranges fights like Jutland.