[Crystal_Ageha]

When I think of exceptional voices, ayaka, MISIA and AI automatically come to mind. They're amazing! Those are top three, for me.
Mika Nakashima also has an amazing voice, and Kumi Koda is near that level, too. Oh, and that Stephanie chick... Her actual voice is kinda eh, but she can hit whistle register - and it seems perfectly~ controlled - so that's impressive. alan is similar to her, too, though not as good, in my opinion. And Angela Aki, she has incredible lung power and pretty good range; a very balanced, very polished singer all around. Sachi Tainaka is an underrated singer, probably since she usually sings anime songs. But unlike the other anime singers, she actually has an amazing range, and she sounds great live.
Hikaru Utada has such great potential, but she often sings out of her capability, and she strains too much. It sounds really bad a lot of the time. So she drops down a few spots because of that. As for BoA.... She's an EXTREMELY overrated singer. The thing about her is, her actual voice is just powerful. Her actual singing is just slightly over average, at best.