For those saying Namie can't sing - you obviously haven't heard some of the rare gems in her discography. If you can listen to I Will, Wishing on the Same Star, Rainbow Moon, Wagamama Wo Yurushite, and Shine More and tell me she can't sing then you need to get your hearing checked.
And she sounds fantastic live. You try dancing your butt off and then popping out some of the notes she has to and then you'll understand.
Comparing her to Britney Spears is just ridiculous. She could sing Britney Spears under the table.
In my own opinion, Utada is the best to me out of those listed. I don't really see how her music is boring
. Heart Station wasn't the most exciting for me, so I'll give you that, but I even loved that album. Anyway it's not about the music - as was previously mentioned - but the vocals. Utada has an extremely powerful voice. Say what you want about her range, but her VOICE is powerful. In most of her concerts you can't hear her audience applaud or cheer because that's how overpowering her voice is. Also -despite what some have said - there IS maturity in her vocals. Go from Heart Station to 'Me Muero' on This is the One and then to Show Me Love (Not a Dream) on Singles Collection Vol 2 and you can hear it (and elsewhere).
I don't really see Ayu's vocal progression as maturity. Instead I see it as 'these are the only notes I can hit now days so let's see what I can do at this level' -- she was practically screaming all throughout 'Love Song' (not the entire album though). Though I still love her so i'm willing to look past, lol.
I am NOT a fan of Ayaka but I will say she has an amazing voice. Her vocals are very smooth and powerful. 'Why' is such an amazing song.