I'm only counting people with more than two studio albums, cuz it's EASY for me to say alan or abingdon boys school or school food punishment or immi or furukawa miki, but honestly, they don't have enough material for me to really know for sure yet.
Ayumi Hamasaki (yes, even with Picaflor's criteria. I'm crazy! XD)
Garbage
Tori Amos
Utada Hikaru
B'z (yes even their old dorky stuff, I don't know why I like it but I do)
Do As Infinity
Faye Wong (yes even her old crappy r&b stuff... see B'z
~)
The Beatles
Fleetwood Mac (...honestly, I feel like Fleetwood Mac (Lindsey/Stevie/Christine/John/Mick) is a different group than Peter Green's Fleetwood Mac... so I'm not sure if this one counts or not)
Death Cab for Cutie
Alanis Morissette (even her very cheesy canadian dance/pop albums. Again, I'm crazy.)
Aikawa Nanase
Anggun