The greatest champions outside of lane to counter Aatrox include Kalista, Aphelios, and Samira. These champs are all good at countering Aatrox with win rates over 54%. Even if these Aatrox counter picks are not played in the same position as you, it's not a good idea to choose him into any of these matchups. They will often crush Aatrox.
Conversely, if the opposing team choses either Shaco or Taliyah, Aatrox mains should strongly think about taking him, as he easily wins against these champs with win rates over 53% against each ofthem.