The greatest picks globally to counter Darius include Kalista, Lucian, and Samira. These picks are all good at countering Darius with win rates over 57%. Even if these Darius counter picks are not taken in your lane, it's not smart to take him into any of these matchups. They will likely win against Darius.
Conversely, if the opposing team brings either Karthus or Vel'Koz, Darius mains should seriously consider taking him, as he easily beats these selections with win rates over 51% against each ofthem.