most jpop singers sux live...
But if I was going to say who have big voices, I would go for Hitomi Shimatani, Kokia, Angela Aki and Ayaka Hiakara... even if alan and Stephany have good voices, they lack control, Utada sux live, so does Koda, Ayu, Namie, Mika, BoA... Sometimes they can impress, but most of the time they are just average or slightly above it... Never heard Misia live, so I can't say anything about her^^
Ayaka Hikara
http://www.youtube.com/watch?v=L9UWh780V7Y
Hitomi Shimatani
http://www.dailymotion.com/video/x4y...004-tour_music
Angela Aki
http://www.youtube.com/watch?v=srrLospeLdo
Kokia
http://www.youtube.com/watch?v=0_n6yUTMl50
And if we go for older acts, Akina Nakamori
http://www.youtube.com/watch?v=zXmR7KofTpU
Alan is good, but she needs some work to do... But she is young, so, she still has the time ^^