Fiora works well but don't be greedy, if you over extend at level one you will get wrecked, but after level 3 you have the edge (both I and someone else posted this same point)
I already theorised Panth as a good counter, but not theories, actual examples, Ekko struggles against Ekko lol (obviously only in TB or normals). I've only had 4 games, 2 wins, 2 losses. One loss I was Ekko support/adc with shaco, so that was no suprise, the other I was mid vs Fiora and kept getting ganked by Dianna pre 6. Shut him down early would be a good example in that case, people say why ekko when you can dianna, dianna is much harder to kill and has a more reliable instigation dash, she was blowing me up in 2/3 hits, I didn't even have time to hit ulti button if she came out of a bush or something.
Ekko doesn't scale well with mana regen or MR items like some champs can (like Galio and Anirva for example), so he typically builds pure damage. He only really has one significant HP option (crysal sceptor) and two potential armour items (Iceborn and hourglass) making him actually weak against burst mages. Malz destroys him, Victor destroys him, LB should, but he might outplay her.
If you don't kill with your ulti (or at least stun) you are dead
the shield from W leaves before the stun, so don't stand there thinking you will tank damage, finish them or run away.
Have leona or some other CC tank follow his shadow while you engage him, and he will be screwed no matter what he does (his ulti is his biggest damage despite most talk, without it, he is sustain damage not burst, so you can out duel him, just do so in less than 4 seconds or prepare for an ulti on the spot!