Best looking Skin to date?

Spirit Guard Udyr. Also known as Spiral Power Udyr and Lordgennome Udyr.

Arcade MF, someone put a lot of love into that skin

My personal favorite is probably Battle Regalia Poppy. It's extremely simple (I'm not certain it's not just a recolor actually) yet really pleasing visually.

If I'd been asked this question 3 weeks ago, I would have said for in-game it had to be Silent Night Sona, without a doubt or second thought. Her robes were breathtakingly beautiful, the deep near-magenta reds and intricately-detailed gold patterns were simply stunning and the violet, I can't emphasize how amazingly gorgeous the violet was. The gentle blue into lovely gold of her hair suited her perfectly and her golden Etwahl tied it all together. An aesthetic masterpiece. I feel a sad pain that I don't get to see it anymore, not just because Sona is now unplayable, but also because it's just not there anymore.

Don't get me wrong, the new version looks okay in its own right, but it's comparably a weak knock-off. The colors of her robes are a bit bright and the magical violet is completely lost. Her hair has become a dull pastel and somehow looks greenish, like they lazily added yellow on top of the blue. The Etwahl looks like a yellow plastic children's toy. I wish they would've put half the effort into her character models as they wasted on visual FX just so she looks like she's doing something, even though she's not. So I guess I just don't care anymore, I guess I'll just say something slutty like Kitty Kat or Bunny Riv. Blood Moon Akali looks nice too.

As for splash-arts, that's not as clear. There are a lot of nice ones, but no singular one that stands out like the old Silent Night used to for in-game. Dryad Soraka is lovely, I'm partial to Pool Party Leona and Chinese Pharaoh Nidalee is very nice too. The actual list is a lot longer and those might not even be my top three. I guess that goes to show they have multiple talented artists working on them and each one shows off their unique style.

"headmistress fiora" or "definitely not blitzcrank"

I really like both mecha khazix and the new rumble

Lollipoppy

My Favorite skin is Firefox Ahri because of the particle effects above her tail, the heat waves. Really well done skin.

Mafia Jinx is awesome, her dance with the gramophone is amazing.

Mafia/Secret Agent/Arcade MF are all cool.

All of the PAX skins are really cool looking.

And the Arcade Skins as well. All cool. I love the pixel art animations on Sona.

This game has a lot of amazing skins, there are way too many to list.

Blood Moon Akali. Good Lord. I can't even play Akali and I had to get that skin, because LOOK AT IT.

Honorable mention to Arcade Sona, which I bought for similar reasons. And it was not cheap, no sir.

Depends on the criteria.

Midnight Ahri is my favorite though some of the newer ones are better in terms of technical execution and quality of skin textures.

Super Galaxy Rumble!

Arcade Miss Fortune. A great splash art and a great skin. From the sound effects to the splash art to her recall, everything is just fun!

Red Baron Corki. Fell in love with that skin on first sight and bought it even though it was totally overpriced at the time. I could never get used to Corki though so it only comes out in ARAM.

Dragonblade Talon is my favorite or King Tryndamere

Underworld TF is such a beautiful skin in my opinion.

metal gear yasuo

All of Akali's skins ingame (except for Nurse and All-Stars). Of the splashes, Crimson and Blood Moon are the best.

...I don't even play Akali.

Ghost Bride Morgana and Frost Queen Janna.