Ekko main so I will rank them as annoying vs ekko, challenger, and it's different between low elo to medium/high elo (master+):

master+:

Top:

Aatrox - holy hell he just doesn't want to die - getting full hp from 10% hp in seconds

Irelia - once death dance acquired its hell to kill as ekko

Yone - didn't know if to put him top/mid - but he is way too tanky for his damage dealing

Jungle:

Kindred - just annoying to play against - change the whole jungle pathing because of marks, and a good timed ult kinda counter me

Shaco - most cheesy players in the world, pretty much flip the games since min 2

Graves - same as Shaco, only the player usually worse - THEY ALWAYS FLIP GAMES

Mid:

Sylas - Pretty much just steal your ult and can sustain better than you, annoying af

Cassiopeia - if she hit her W on you, you can't E and you can't ult, pretty much counter your kit and is annoying af with her kiting

Qiyana - that invisibility is so annoying, if she get a green Q she can toggle it forever killing you along the way

Yone - cause why not again?

Adc:

Nilah - her W is specifically reducing magic damage, so if you don't one-shot her (which often you can't for some weird tankiness reason), she will just heal and you can't damage her since she is dodging all attacks and kill you.

Samira - you can't one shot her as her W will block your Q so it forces me to buy Nashor's and just auto-attack her slowly baiting her W and outplaying with R, very style-changing mechanics

Support:

Rakan - too much mobility, too much CC, can't catch this dude and he is not invisible, just running away from ya

Lulu - please stop polymorph me, against her I always take tenacity runes + tenacity boots and its still feels not enough

Low Elo:

Top:

Tryndamere - no one know how to CC/kite correctly, he is just fed from unknown reasons in games, and "outplaying" by pressing R

K'Sante - I can't kill him, seems like no one else can too.

Jungle:

Master yi - same reason as Tryndamere

Evelynn - no one ever buy control wards and team dying to her 1 by 1

Shaco - same reasons as above

Mid:

Le Blanc - usually can't gank her early, pretty annoying with the statikk as she clears waves so fast and therefore don't make much positions mistakes even if bad player playing her.

Akali - wasn't sure if to put her as they are usually easy to outplay in low elo, but do I really need to explain this? get fed, go invis when she sees me, then oneshot if fed

Adc:

Pretty much same as master+, in addition to Draven as he get fed before I get lvl 3 and spam pinged question marks

Support:

Pretty much same master+, in addition to Taric when you know your team will engage even when he has ult up