Even the strongest ship is worth nothing before it is put to service and already for this reason I would hesitate to nominate ships that weren't in service at all (like) Lion, H-class) or the ones not in service before the war was won (like Iowa class). And even in a clean tech-spec comparison, ships from different generation will say more about the available tech at the given generation than about the design.
But if trying to compare the balance involved in choosing between time and technology I think the Littorio, Richelieu, KGV and Bismarck deserve honour. The Bismarck class dictated British Building hurry, tied a major factor of the Home Fleet and without the Bismarck Class the Japanese probably wouldn't have been able to overwhelm Singapore. Both Bismarck and Littorio were well over-weight however - that detracts. A lot of people have pointed to all the weaknesses revealed in Bismarck when she was shot to pieces by the RN. Sure she was, but no other ship was exposed to similar punishment so we really don't know how they would have fared under similar circumstances. Another ship hit by many (smaller) shells was the SoDak. She didn't sink (the Germans claim Bismarck was scuttled) but was practically blinded by cabling being cut outside the citadel and without North Carolina showing up she probably would have been sunk. An armour scheme on SoDak similar to the much criticised on Bismarck probably would have fared better at Guadalcanal.
Richelieu is in many ways very impressive (and handsome), but had serious dispersion problems until her US refit - a ship that can't hit is not worth much, and much the same can be said about the Littorio class, they just didn't have any US refit.
The KGV certainly was in service at the right time and place, and the Price for being that, her 14" main armament apparently was strong enough. The reliability problems seen in PoW and later DoY are IMHO misunderstood. Any ship as new as PoW could/would have suffered similar problems, and DoY did prolonged firing in very heavy seas - we really don't have similar situations with other ships, and when firing under more normal conditions the misfires on KGVs wasn't different from other ships. Only the Yamato was better protected than the KGVs, but at almost double the tonnage.
The NC class was in service too late to have been of any use to the RN, but in return got a very strong 16" armament and a relative weak protection - and initially serious vibration problems. The time in service wasn't a problem for the USN, but I thing a design with 14" guns and better protection would have been better balanced.
The SoDak deserve special honour for the unique way her machinery was arranged - simply ingenious! For the USN she was with when the going still was tough, but for most others it would have been way too late.
All in all? KGV!