For me it's Battle Boss 
The colour, the X on the face, the smile on the splash art, the fire on top of the head that I don't even know the fuck it is. Even the sounds.
I know it's a well done skin, but I just find the design to be agressively ugly.
true damage ekko
Pajama Guardians << this is the equivalent of investing heavily in some company, but missing your projection completely Many Dark Star << many will disagree here, but I find most of these skins are messes of purple and black... I can't tell what's what Battle Boss Yasuo << actually ugly. Great effects though. Victorious Aatrox << nuf said Pulsefire Thresh << great effects, but wtf is his head Lancer Hecarim << an ok skin, but 10 gems? And his attack doesn't even use his lance point True Damage << Yes I'll go there. Just so boring. Maybe it's for the young hip millenials, but I just dont care for this line at all.
-preps for downvotes-
All the True Damage skins. They just look like stereotypes squashed together into a mess of something, but I know that the quality level put into them is high. Maybe I just don't like the music lines visuals since I didn't like the looks of the KDA skins either.
By 'good quality' I'm going to assume you mean 'made recently'.
True Damage Qiyana. I just find the whole Nicki Minaj aesthetic ugly. Splendid Staff Nami: It's not Nami. It's some chick with a fake mermaid tail attached. All of the IG esports skins: If all you showed me were their heads, I wouldn't be able to tell a single one of them apart. Horrible aesthetic. Elderwood Ahri: Oh wow it's another skin where Ahri's not wearing much, except this aesthetic just looks completely awful on her. Project Jinx: Project skins are really hit and miss. Jinx's misses the target more often than I miss ults with her. Battle Academia Lux: Same thing with the IG skins. If I wasn't told that she's Lux, and all I saw was her head, I couldn't tell you who she even was. Dunkmaster Ivern: Just... why is the only skin Ivern will get in three years a skin that has no connection to him whatsoever? Blood Moon Aatrox: Just looks horrible in comparison to all his other skins. KDA Evelynn: While I'd love to point out at any time possible how much I can't stand kpop('s American fanbase), I don't mind the Ahri, Kaisa or Akali skins. Evelynn though? This skin line is supposed to show a little sex appeal, yet this is probably her least sexy skin. Enduring Sword Talon: I find this skin line extremely bland, probably the blandest in the game. This skin in particular is even blander than all the others. Pool Party Zoe: Just ugly. This skin also came out at the height of my utter contempt and hatred for this champion, so there's more than a little bias there. GG MF: It's okay I guess. It really shouldn't have been an ultimate skin though. Super Galaxy Gnar: Like Pool Party Zoe, it's just noticeably far uglier than the rest of the skins that came with it.
Far too many
Victorious Aatrox. Very good ability effects but the skin is so damn ugly! Makes me wish I hadn't spend all that time getting plat in Rift and TT Dragon Oracle Udyr. Such a good skin but the thing that makes it suck is the base animations that were not changed at all Blitzcrank. Both Lancer skins are exceptionally good but again the base animations which they didn't bother to change even tho these skins are expensive And Blood Lord Vladimir. So much lost potential. Could have been such a badass skin but it ended up..meh. The quotes are very few, the ability animations are the same basically and the death animation is not good and extremely buggy
papa jon's pizza
Fuzz Fizz is one of those skins i find hideous
Elemental lux Dj Sona Literally 90% of the hextech skins.
Dark Cosmic 
It is a very well made skin but so ugly to me.
