I've been fortunate enough to see Ayu's last 3 tours, and have also seen:
Perfume - Disco!Dicso!Disco! last year. That was a lot of fun
Kumi Koda - Universe tour this year. My favourite concert
Utada - in London in February. It was a much different atmosphere than a concert in Japan. Really enjoyed it though.
I'd still like to see Namie at some point, and attend a CDL. YUI would be good too.