The top champs out of lane to counter Diana include Caitlyn, Aphelios, and Zeri. These picks are all good at countering Diana with win rates over 55%. Even if these Diana counter picks are not chosen in the same position as you, it's not smart to take her into these matchups. They will very likely destroy Diana.
Conversely, if the enemy team brings either Karthus or Amumu, you should seriously think about selecting her, as she easily destroys these champions with win rates over 54% against both ofthem.