The discussion is an interesting one - although we know who won the war, the Japanese fielded several interesting and highly advanced craft. For their time.
Among these were several submarines like the I-14 and the I -201 and indeed the I-400 class.
Arguably these were some of the more interesting designs of the war - although operational use didnt amount to any major advantage for the IJN.
I enclose a link that is to be taken by a grain of salt - but indicative, still.
It is worth to remember that the performance of the IJN was hampered to some extent by rigid lines of command and doctrine that was outdated by the time the massive US airpower could be brought to bear.
http://www.combinedfleet.com/ss.htm