Sorry for bumping an old topic but Japan have "mass produced" world class boxers on and off since Yoshio Shirai. They sadly just don't get much respect in the West.

As others have said Harada was probably the best. Though others to think about (and at least give a mention to would be):
Hiroyuki Ebihara
Masao Oba
Kuniaki Shibata

Plenty of others have left their mark on the sport though.

Unfortunately a lot of Japanese fighters are totally ignored by the western boxing press which is one of the most shameful things in the sport.