I'm going to second everybody who said
Koda Kumi. She is amazing. If you haven't heard them already, I would suggest
Pop Diva,
show girl,
TABOO,
Hot Stuff,
JUICY and everything on her
JAPONESQUE album.
I would also recommend
KAT-TUN. They have been my favourite group since 2008. Maybe they won't be your style, but they've a large discography (nowhere near as massive as Ayu's though!) but I would definitely recommend checking out some of their latest songs (2010~2011):
CHANGE UR WORLD,
MAKE-OR-BREAK,
ULTIMATE WHEELS,
PERFECT,
Yuki no Hana,
RUN FOR YOU,
COSMIC CHILD,
STAR RIDER and
LOCK ON. Their older songs are good, too, but I find every release more addictive than the last - although my overall favourite song by them is
Yorokobi no Uta.
