The greatest picks outside of lane to counter Kog'Maw include Elise, Naafiri, and Nidalee. These champs are all great at countering Kog'Maw with decent win rates over 61%. Even if these Kog'Maw counter picks are not picked in your lane, it's not a good idea to play him into any of these matchups. They will most often crush Kog'Maw.
Conversely, if the opposing team choses either Rumble or Skarner, Kog'Maw mains should seriously consider taking him, as he easily destroys these selections with win rates over 60% against both ofthem.