So judging based off this video and watching numerous live performances of Twice my ranking is...
1. Jihyo( highest vocal range and most stable)
2. Nayeon( great at projecting her voice)
3. Jungyeon ( has a nice vocal tone and projecting her voice but gets off key and cracks at times/ unstable
4. Mina( nice vocal tone but has trouble with singing louder than the backtrack) and Sana( great projection and is very stable.
5. Chaeyoung( Very little videos of her singing to really judge but from what I hear she sounds really good and very stable) and Tzuyu( Has improved a lot and sounds very good in her comfortable range but tends to crack)
6. Momo and Dahyun( weakest in the group but don't sound terrible, I see them improving a lot when they sing more often in their comfortable range a.k.a not giving Momo a high range that she isn't good at)