Ikuta, Kaki, Kubo. About Kubo, go see Nogizaka Star Tanjo ep 2x02, see sings "I am proud". For me it's her best exhibition. I like also Asuka, Naachan and Junna. But here it's about personal taste. I prefer voices with good timbre than technical perfection. 5th gen is surprisingly good at singing, in general.
S tier >> Mahiro, Iku A tier >> Junna, Reika, Ami-chan senpai B+ tier >> Kubo, Misa, Chii, Aruno, Yumiki, Runa, Karin, Yuna B tier >> Kaki, Sayaka, Nagi, Seira, Riria, Kaz, Iroha
Ikuchan, Kawamura, Misa-senpai, Kazumin, Ito Junna, Ito Karin, Kubo, Yuna, Runa, Sayaka, Aruno, Nagi, Iroha. Special mention to Manatsu and Sayuringo
Rotti and Ikuchan as a lot of people have said above I love that they're completely different singers though, Rotti's voice is more recording artist while Ikuchan has a musical theatre-style voice
1ki: Ikuta 2ki: (I don't have clear opinion for 2ki bcs I haven't seeing much but I like Sabita & Atarashii Sekai, so Renachi & Ayane) 3ki: Kubo 4ki: Runa, Yuna 5ki: Aya, Mao, Aruno, Nagi, Iroha
1ki : Ikuta , Reika , Misa 2ki: Junna 3ki: Shiori ,Riria 4ki : Yuna , Runa , Kaki 5ki: Nagi , Mao , Maruno , Aya , Iroha