I based my ratings extensively upon the work of Nathan Okun. From his paper detailing the usage of Bismarck's 15"/47 gun to shoot at all seven of 'The Contenduh's', I extracted a quantification of the total zones of vulnerability, for both deck and belt armor, of each of the seven ships. If you want the really gory details on how I did this, click here. Suffice it to say that I am surprised as you that Iowa has the most effective belt armor of the lot; I would have bet on Yamato any day. But Iowa's combination of an inclined belt, and a highly effective STS-steel shell plate outboard of the belt (which has just enough resistance to strip the AP cap off of an incoming shell) tips the score in her favor. Richelieu also had this same design, and very good protection as a result. Bismarck, despite the reputation of her side armor, fares very poorly in this category. From a deck armor perspective, Yamato comes out on top, followed closely again by Richelieu and Iowa. Vittorio Veneto is very vulnerable to high-angle fire, and Bismarck is as well. Yamato thus emerges as the best armored of the lot, followed closely by Iowa and Richelieu. This makes perfect sense to me, as Yamato also had the distinction of carrying the only armor plates which were completely impervious to any battleship weapon ever mounted afloat -- her 660mm turret faceplates. She was, indeed, an awesome beast. It makes the American and French feats of achieving protection within a hair as good, on much smaller displacements (particularly the South Dakota, which has the second smallest displacement of the seven warships detailed here), a very impressive feat as well. On the bottom of the heap, Vittorio Veneto and Bismarck were both penalized for their inability to cope with a long-range gun duel. Bismarck also suffered from the poorest belt armor of the lot.