Best way to counter Mordekaiser ADC...?

The reason Morde ADC works, is because of his support. He wouldn't be able to do jack shit if he has a Lulu, or Janna support, for example. Most Morde's play with a Leona, a champion designed to keep someone stuck in place, with high initiative, thus, Morde can do his DPS. So, the only thing I can think of, is to ban Leona, Naut, etc.

items and champs that can reduce healing... out poke him and take ignite

% hp damage helps too

support if you have a tahm he works well vs mord with his W to save the adc and his % hp damage

varus with poke. and his E for slow and reduced healing