Want to beat Ekko, the latest QQ champ? Play some Swain.
I haven't played against but one Ekko on live servers, but on the PBE, I just had to try out my main against the new guy on the block.

Granted, it was just one game, but even theory-wise, Swain is strong against Ekko, especially once he finishes an Abyssal Scepter.
Ekko excels at close range burst. His primary engage and damage tool is his E, which let's him close in on a target, after landing both procs of a Q. From there, he does his damage and either ults or walks away.
Swain can take huge advantage of this. Swain excels in close range duels because of his incredible sustained DPS and tankiness. Even if Ekko gets his full Q+E+passive combo down, it's nearly impossible for him to burst down Swain at any stage of the game unless he gets super fed.
Ekko goes in without his ult? Easy snare by Swain. Ekko goes in and has his ult? Swain can easily force him to pop his ult to get out of range of crows and his built in DFG.
Give it a shot! I know I'll be having fun against any Ekkos that come my way.