The best champions globally to counter Nami include Kalista, Aphelios, and Zeri. These champs are all good at countering Nami with decent win rates over 55%. Even if these Nami counter picks are not used opposite your position, it's not smart to play her into any of these matchups. They will often win against Nami.
Conversely, if the enemy team brings either Cassiopeia or Karthus, Nami mains should strongly think about selecting her, as she easily dominates these selections with win rates over 54% against each ofthem.