I do feel like J-Pop was at its "best" - in an objective sense - between 1999 and 2003, because in there, the melodies still had the nostalgia of older J-Pop, but thanks to more globalized attitudes, production was finally advancing beyond what TK learned when he went to London in '88. (I just wish we'd had more low, soulful voices on the charts at the time than just Van Tomiko and Utada Hikaru, and, at the tail-end of this, Asuca Hayashi. Vocals were still pretty weak in general).
It was when that globalized attitude started affecting J-Pop melodies in about '04-'05 that J-pop started to go seriously downhill for me. When I got into J-pop I feel like it was the best possible blend of what was great about western AND asian pop, and now it's just outdated western pop in another language.
The INSANE run of cover albums that have been released lately - that for once are covers of Japanese songs rather than Western ones - are signaling a backlash coming, hopefully the nostalgic sorts of melodies will make a comeback soon. I think a bunch of singers are dissatisfied with how J-music sounds these days too.