The best counter-play is poke, lock-down CC, QSS (Clears his stacks and various CC).
Another thing is kinda common knowledge for champs, but don't chase him.
Chasing anyone is a terrible idea, even more for Ekko who's ult is an oops / re-position / nuke / damage mitigator / skill avoider / oops i did it again type of skill, and his Q with 40% cdr can proc his passive all by itself, meaning free speed for him and tons of slows for you. Ekko's are very easy to read once you understand his kit, and keeping track of his ult is just as important for Ekko as it is for the enemy team.