>>83237149You took the time to put them on a pic, so I'll answer seriously. Since your question is 'who is the best singer', it's important to separate that from 'who makes the best music'. Second, the best way to establish any kind of ranking and shut up most of the arguing is to crown the best of each branch.
JP: Suisei. Shouldn't need much of explanation. AZKi used to be a contender, but she's getting older and has lost her intensity and some of her dynamic range. Suisei after all her throat surgeries is also not what she used to be, but she's still kept more of her edge than AZKi.
ID: Risu. Moona makes better music and would be the better choice for a power-singer group to perform on stage due to her style, moves and presence, but Risu is the better singer. Not by much, but it's there. Also worth noting that Kobo is right behind Moona. ID is the Generation of Miracles for singing.
EN: IRyS. Controversial since she makes garbage music 9/10 times. But she's the better singer. Nerissa is the product of work, not talent, it takes deliberate effort for her to sing well and when she struggles even slightly in karaoke and loses concentration her performance falls off hard. Liz is the opposite, she has the talent but no skill. She's a football national anthem singer that butchers every song that exits her lips. IRyS has both, it's as easy for her as breathing, she just wastes it with the wrong producers.
Overall: Most likely Risu. But the others are more professional at being singers, there's a reason she's not in Star Flower.