The top champions globally to counter Dr. Mundo include Zeri, Tristana, and Caitlyn. These picks are all good at countering Dr. Mundo with win rates over 54%. Even if these Dr. Mundo counter picks are not used in your lane, it's not smart to take him into any of these matchups. They will most often destroy Dr. Mundo.
Yet, if the enemy team picks either Galio or Rumble, Dr. Mundo mains should strongly think about picking him, as he easily destroys these champs with win rates over 55% against each ofthem.