i voted for
ayu - she may not always sound great, but for the most part, her performances are fun and really engaging.
kuu - she tends to sound quite good live, and she dances really well too. she has a lot of interaction with her crowd, and she just seems like fun!
bonnie pink - usually sounds great. she doesnt always have the best crowd interaction, but when she does, she's sweet and fun.
But really, hands down, best live performer is Natsukawa Rimi.
She amazing. Her voice is actually better live than on cd. The power, strength, emotion and just beauty of her voice is abosolutely phenomenal.
Watching some of her live videos on youtube, on that crappy little format, she has nearly brought me to tears.
She's just amazing.