It still depends on what you wanna DO with a sub.
For taking out other subs the KMSS5 is the best boat, followed very closely by the KMSS4.
For taking out BBs nothing beats a good IJN SS4, the HEII. The IJNSS5 has long range ability, but it's dependent on your team's CVs and their abilities.
The Gato, MNSS4 and the UKSS4 are good "all around subs". They're not the best at anti-sub or anti-BB play, but they come close to being able to do both.
