Renji (solos) takes this with mid to high diff, until we see more of Byakuya.
1. Shikai Renji is around 10x times more powerful than average captain fighter.
2. Byakuya before RR, was no more than average captain fighter.
3. Byakuya's current shikai is just strong as his previous bankai, which mean that in his current shikai he is no much above average captain fighter.
4. Add on top of that their bankais, and difference is still the same.
5. Not to mention that Byakuya's powers were more mature before RR and that he mastered his bankai before going to RR, so Renji had more room to improve.