I would have thought someone in the Holy Trinity (Taeyong, Mark, Jaehyun) but I think SM is actually going to do the right thing and give Taeil his solo first (he's the main main, one of the best voices of his generation and he will be enlisting in '23 - he should be first). The success of his OST in Korea has to have given him a boost up in getting his solo first.
Then it will be one of the others - probably TY or Jaehyun.
Ten will be the first soloist in WayV - SM has made that abundantly clear.
For Purple Kiss, it will be Swan, who is already doing solo-related work.