Many of my faves have already had solo debuts out of their groups, so of who is left...
NMIXX: Kind of a toss up on whether Lily or Haewon will go first, but I'm most interested in Kyujin.
Fifty Fifty: I think Chanelle is most likely to go solo first, and I'm most interested in her solo too. Hers or Athena's. Athena has a nicely distinct voice.
A2O MAY: I would bet LSM actually does a twin unit first (Kat & Miche), but I'd love to see Quchang. Out of their solo B sides on their debut album, I was obsessed with hers.
For my other favorite groups, I don't have a strong sense of who might go first or have a strong preference. And of course, I wouldn't be mad if someone else went first. It's just who I think I'm most likely to be sold on as a soloist.