What about option "Ayu is the best for me"?
There's no one better than Ayu - there's just many other great artists as she is. How to say who is better - because someone sold more CDs or it's prettier or what? That's silly!
The thing is everyone have his/her own taste, favourite sounds and ellements in music.
Different artists singing different styles and have different fans. You like Mika Nakashima and that's ok, she is great singer but not everyone loves jazz and ballads - for many peoples they may be just boring. So what's wrong to like for ex. Koda Kumi, Namie Amuro or Utada Hikaru? Nothing. They are all very talented but DIFFERENT. I like all of them - some more, some less. I like also some other singers from your list and some which aren't there. However Ayu have special place in my heart from many reasons and she doesn't have to be "the best" for anybody else - you can call her the worse singer in the world but she'll be still the most precious for me.