The top champs globally to counter Zyra include Lucian, Draven, and Xayah. These champs are all great at countering Zyra with decent win rates over 54%. Even if these Zyra counter picks are not picked in the same position as you, it's not a good idea to take her into any of these matchups. They will often crush Zyra.
Conversely, if the other team brings either Rumble or Karthus, Zyra mains should strongly consider picking her, as she easily wins against these champions with win rates over 56% against each ofthem.