Friedrich der Große
Not compleatly invulnearble to citadels
When Friedrich gets close to an enemy ship her turtleback armor is supposed to protect her form taking citadel damage, this is 99% true, it works perfectly against Destroyers, Cruisers (both CL and CA) and Large Cruisers; but in case of a close range brawl with Iowa the ship presents a vulnerable area just below Main Turret 1 that if targeted by Iowa and her formidable AP shells, Friedrich der Grosse can sustain a citadel hit, not more than 1 because of the 30 second reload of Iowa Main Guns, and because of other factors like Iowa's speed and RNG. I don't know if there are other Battleships that can do this, the only ones that I tested for this are Iowa and Georgia, I assume that Montana can pull off this stunt because it's just a slower but more powerful Iowa (both ships have the same Main Guns 406mm/50 Mk 7), it is possible that some of the Big Guns like Kremlin, Ohio, Yamato and Shikishima can do the same thing, but I haven't tested yet if it's possible with those ships; I'll update as soon as I'll find it out.
Update on previous entry
Good 'ol Friedrich will sustain Citadel hits (see previous entry for the weak point) even from big guns Battleships like Yamato, Kremlin, Ushakov, Patrie and Satsuma (it will probably be the same with Shikishima, but since I don't have access to that BB I can't say, based on Main Gun diameter, I'd say yes, since the Main Guns are the same of Satsuma), but it won't take citadel hits from Georgia, and for Ohio it's the same; but it won't sustain Citadel hits from Preussen and Hannover, because of the horrid Main Battery Accuracy (even when boosted), and it won't sustain Citadel hits from Tier 9-10, British BBs and BC because of thier short fuze AP shells.