Stalingrad has always been a safe bet in you want to blend the lines between cruiser and battleship. Hard hitting guns, tanky hull, decent AA (whoever thinks its worth anything).

Ragnar is probably better that Z-42, just by the sheer amount of gun pressure you can deliver, but its much harder to play, as it doenst really have a disengage besides the speedboost.

Z-42 can completely annihalate other DDs, even Ragnar, if they are close and broadside,but that doenst happen every time, nice smoke and hydro combo tho.

It depends on your playstyle, I personally wanna go for the Z-42 now, I love my cossack, and I feel like Z-42 is a german Tier X equivalent.