most of you know i'm a HUUUUUUUGE Airi fan. my bby.
but i can see what he means. Airi is a fantastic singer, but there are things Ayaya (as much as i don't personally like her voice tone, i concede to her ability) and Aichan can do with their voices that Airi can't, although i'm not sure if that's due to her voice type or her ability. i'd put her in a more similar category to other great H!P singers such as Reina and Maki. still great, but not on the same level as Aichan and Ayaya.
Mikitty, on the other hand, should have been mentioned by Tsunku. i don't think it's possible for Sakura to pass her on anything except perhaps diversity in the styles she can sing in, and that's probably only because i haven't seen enough from Miki as she was not a soloist long enough to get an extensive discography of her own, and didn't stay in MM long enough to get Tsunku to start lavishing her with solo songs in albums and concerts.