Last game I was playing Ziggs vs Ekko. He countered me so hard that I had to take it in the butt multiple times because he had too much mobility and damage.
But let's ask the big question here - who truly counters him? I can imagine squishy ranged casters doing a terrible job vs him because they're squishy, but assassins are also quite poor against him due to his health reset on his R. What about tanks then?
The way I see it is that Ekko is like a combination of Ahri and Fizz with more CC, more mobility, and a "get-out-of-jail-free-card" on his ult.