Well I haven't been following her forever, so I can only pick songs from albums which release I personally been through
So for me:
Mirror
GUILTY was the first album I bought of her, right at the release and I had really anticipated it. I remember the moment so well.. putting it in the CD player and starting the album with mirror.. goosebumbs!! That was so awesome, I still love it. Of course I also love Mirrorcle World but mirror started it.
Sparkle
From the moment I heard it on the Honda commercial, I was in love with it. It's very electronic, something she hadn't done in a long time. Still love it!
LOVE 'n' HATE
I don't exceptionally like this song, but it's very different! Very refreshing. I like listening to it, but it's not a favorite.
Don't Look Back
Need I say more? It took GREEN's oriental flavor and brought it to a whole new level, I loved it.
Sexy Little Things
This song is so weird, and so, so , SO catchy! I can't stop listening to it!