I'd love to throw ZARD aka Izumi Sakai into this mix - she was amazing songwriter
I first heard of her through her presence in the DragonBallGT anime and it's through her that I got interested in JPOP - Ayu came soon after and made me jump straight in, but Izumi has a special place in my heart
I've often seen her song "Makenaide" be referred to as the song that helped Japan out of the post-bubble depression - I can see why, it's got a positive, upbeat look toward the future that can't help but make you feel confidence. Kanariya, Get U're Dream, Don't you see!, Mind Games...they're all wonderful.
ZARD's lyrics are here:
http://zardthebest.tripod.com/translation.html
And for worst, right now I'm feeling Lady Gaga post-Fame Monster and Katy Perry/Ke$ha....lol