Damn that's a great question.
I truly don't have a favorite. I love some skin lines
The JRPG skin line aka "Rift Quest"
I'd include Master arcanist Ziggs in that line, no clue why riot didn't.
The toy line, which i would also throw in the cats vs dogs.
out of these i mainly like the Dino Gnar type skins, Meowkai, Pretty kitty rengar etc
Something about these scary killers in cute onesies that really make it.
The only skin line that i find ugly or lacking enough nice skins is infernal.
Other than that each skin line has something awesome in it.