I would say that it's between AAA and MLB in terms of overall level. Keep in mind that MLB pitching has dramatically worsened on average since expansion in the 1990s. Certainly the best pitchers in MLB are better than the best ones in NPB, but when you get deep into the rotation in teams like the Rangers or the Pirates, just about any pitcher regularly starting in Japan could do better. If you were to swap all the Hanshin pitchers (or even Yomiuri, for that matter) with the Rangers pitchers, the Rangers would be vastly improved, so NPB compares favorably in terms of pitching. With position players, by contrast, there are probably some people starting in Japan on the weaker teams that would be in AA in the US. In general, though, I think most NPB teams could beat most AAA teams.
A PanicInducingGwailo since 2009