The greatest picks globally to counter Rengar include Lucian, Xayah, and Aphelios. These champs are all great at countering Rengar with high win rates over 64%. Even if these Rengar counter picks are not played in your lane, it's not a good idea to choose him into one of these matchups. They will likely crush Rengar.
Conversely, if the other team choses either Rumble or Heimerdinger, Rengar mains should seriously consider taking him, as he easily crushes these champs with win rates over 51% against both ofthem.