Might sound selfish but one of the aspects of being into J-Pop (or any foreign music) that I love is, by not being played on the local channels, you've basically got a world of instantly listenable high production value mainstream music to discover on your own and enjoy in whatever capacity suits you. It's amazing how even the most obviously "engineered to be a hit" music remains tolerable for extended periods of time when it's not pushed upon you to the point of over saturation. Personally J-Pop helped me find an appreciation for pop music that eventually extended back to domestic mainstream and made me realize how much quality music I missed out on by being an elitist prick (it's all good though since it all remains available for me to enjoy).
Although I appreciate and occasionally listen to the alternative stuff like SUPERCAR or Quruli, not being anywhere near proficient in the language (more like 3-year old's conversational level) I tend to gravitate more towards the lighter stuff so most of my recommendations will be in this vein.
Bonnie Pink - she's been around since the mid 90s; heavily involved with the creative process (writing, composing); retroish pop/pop-rock with a dash of blues and funk thrown into the mix; fairly proficient at singing in English (doesn't make me cringe); classy, mature aura so dearly missed in todays scene:
http://www.youtube.com/watch?v=7ZAEyMFko7A
http://www.youtube.com/watch?v=VaMJJjpestQ
http://www.youtube.com/watch?v=c7aVRlSV7Bg
Kaori Natori - been around since the mid noughties; rooted in the R&B/urban pop school of music; recently joined the duo
Spontania; fell in love with her attitude and dramatic looks:
http://www.youtube.com/watch?v=wBypyTLYqRA
Ayaka - as far as recent female soloists go this one is a must listen; fairly straightforward pop rock style with an occasional ballad thrown into the mix; awesome powerful vocal complemented by an endearing persona; unfortunately she's currently on hiatus due to illness:
http://www.youtube.com/watch?v=rYkzGXqAoYs
http://www.youtube.com/watch?v=HudqwQV8YDk
MiCHi - british-japanese cocktail (quite a few in Japanese music/ent); fairly recent dance pop hotness; few decent songs, one really amazing one:
http://v.youku.com/v_show/id_XMTU5OTY4NTMy.html
Aya Kamiki - played local shows in more of a blues rock style before being picked up by a label, which due to her looks tried to push her as the Japanese answer to Avril Lavigne; great captivating vocal that fits basically any style she's attempted so far:
http://www.youtube.com/watch?v=BFiDsJ4K8lI
http://www.youtube.com/watch?v=wIeNwNFdsQM
http://www.youtube.com/watch?v=YGb5ld5VXB4
though admittedly her music hasn't been that hot since she switched to avex
Kana Nishino - definitely the queen of teeny bopping girly girl pop for the past year or so; backed by a foreign production team her music sounds "current" even by rest of the world's standard, though the style all too quickly became anchored in the sappy pop'n'b ballad formula (more variation please); very uncharacteristic for a Japanese girl act she's found fandom with the female audience thanks to heavy marketing towards the Gyaru (Gal) subculture:
http://v.youku.com/v_show/id_XMTUzNDEzNTEy.html
http://v.youku.com/v_show/id_XMTM2MTA3MDYw.html
Jasmine - striving to become the Japanese Kellis it seems; I like her trashy side but I'm afraid she just doesn't have the voice for the "big" ballads she's been attempting:
http://www.youtube.com/watch?v=BiGf3vJAudA
There's loads more that I enjoy and would like to recommend if I wasn't in a hurry to catch some z's right now. Will try to think of some dudes next time (
hint).
Oh and as long as we are posting really old (pre J-Pop) music:
http://www.youtube.com/watch?v=EAoPBVeNFAk
This lady subjugates me like no other.