[Rock Steady]

Koda Kumi.
I think she's at her absolute best when she sings live. There are some live versions of her songs that I think are better than the studio version. (ex: Heat @ Black Cherry Tour and girls @ Kingdom tour)

I think that Ayu still sounds good, but she's lost it a little.

And Namie Amuro sounds horrible live. She's got some great songs, but she can't sing live.

Now, if we were talking about male singers, then I would defiantly choose Shou from Alice Nine., Gackt, Ruki from the GazettE, and Park Yoochun from DBSK.