I'd have to go with Rosé, however back when she was trainee she did a song with G-Dragon called Without You and I think she sounded her best there. She used her voice in lower register and she sounded amazing, but she still sounds great though.
I think they all sound good but it was between Jisoo and Chaeyoung for me they just have such unique voices. I voted for Jisoo in the end because I just love the sound of her voice!