The members are in alphabetical order. I got the idea for this thread from this post:
Nayeon by a landslide.
3 mix are the best singers in the group but we all saw how much people hated Jihyo's singing style and Jeongyeon lost alot of confidence due to her illness. She's never even done a melody project. I doubt she'd want solo music.
Chae would drop indie music, Mina and Tyuzu are wildcards (don't think they have the charisma or personality to be solo but who knows) and if the song is right Sana and maybe Momo could do really well.
Yes, the J-line would be better off continuing with the subgroup than going solo.
Nayeon and Jihyo would give solo careers a shot, they may do ok, not as well as IU, but probably better than say ChunHa.
As someone said Chaeyoung may do more indi stuff or collaborations.
Dahyun could do well in variety shows.
Jeongyeon could perhaps try to follow her sister into acting.
Tzuyu will probably go back to Taiwan and try to do something there.
nayeon without a doubt. shes the most famous in korea and one of the most known internationally.
another thing, a lot of the twice members have their strength in one category while being average or slighlty weak in everything else. an example of this would be momo or jeongyeon! momo is very strong in dance and performance/stage presence but is average in rap (well not by technical rap standards since most kpop idols would fall flat 
but she could function as a lead rapper) and weak in vocals. jeongyeon is a strong vocalist but average dancer and has weak stage presence. however nayeon is a strong vocalist, good dancer and good performer, id say top 4 in twice (jihyo>>>>>sana>=momo>nayeon>>>>>>>>chae=jeongyeon mina>=>>>tzuyu)
jihyo is also a good singer, dancer and performer, and really popular internationally, so why did nayeons solo do better? i think it came down to domestic factor. a lot of idols are trying to strive for international popularity only but i think a domestic fanbase is equally as important. sales and touring is good money wise, but streaming/charting helps your song and your face get known in the public (which can lead to brand deals/ads!!!). not only that, nayeons solo debut SCREAMED kpop and was so quintessential twice. (plus was the first solo which was much more anticipated) jihyos solo was on the mature side of things, focusing a lot on vocals, opposite to nayeons bright tunes and catchy hooks. i cant remember a super BIG defining part of killing me good compared to pop (still a good song nontheless)
does that mean jihyo should change her solo image to be more successful? absolutely not! i think the albums should stay true to the members tastes, so i couldnt care less whos more successful.
The rest of the girls have never been soloists. 
Nayeon will define the most popular.
But I think Jungyeon would so really well too. She's just really likable imo and she's a great performer and is gorgeous. I also get the impression Korea really likes her. I remember her ranking high in Gallup back when they debuted and her MC'ing also really helped cement some of her popularity in Korea.
Mina is also really likable and can pull off a cute solo if she wanted to. More of a sunmi career. maybe not full albums, but a hit song and do solo promo.
Chaeyoung is the lowkey ace of the group. I'm not worried about her one bit honestly. She's a girls girl too. Like I see girls really liking her and thinking she's cool and fun.
Sana is good with her endorsements and I can see her as a personality in Japan.
I can see Momo being big in Japan and focusing on dancing and modeling.
Tzuyu of course is set with her beauty.
Dahyun is set as a future personality in Korea broadcasting.
And I actually see Jihyo following Taeyeon and releasing solos and being a personality on some variety.
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