The greatest champions globally to counter Rek'Sai include Aphelios, Samira, and Zeri. These picks are all great at countering Rek'Sai with high win rates over 58%. Even if these Rek'Sai counter picks are not used in the same position as you, it's not a good idea to take him into any of these matchups. They will likely beat Rek'Sai.
Conversely, if the opposing team brings either Vel'Koz or Tahm Kench, Rek'Sai mains should strongly consider choosing him, as he easily dominates these champions with win rates over 54% against each ofthem.