Wendy is on another level. Best vocalist in the world in my opinion.
NingNing and Winter are incredible, love them.
Jennie is a very underrated vocalist. She sings with a lot of soul.
As far as overall groups go, I'd rank them
NMIXX
aespa
BLACKPINK
Red Velvet (Don't kill me, five well-rounded queens, but not all of them are great singers -- Wendy is amazing, Yeri is strong, Seulgi and Joy are skilled but nasal, and Irene seems to mostly rap)
Fifth Harmony
I'm not familiar with Little Mix, so I didn't rank them. I'm aware of some of their singles and they seem to be very strong vocally, I just don't vibe with their concept or music so I've never really delved into their discography. I tried listening to their debut but I bounced off of it hard.