Volibear literally did just come from a community post that said "We need an armored bear" that gained unfathomable amounts of popularity.
Maybe not the most...inspired champion, but as much as I disliked his addition to the game, he's probably not the worst designed. Thematically, many of the early champions were similarly one-note on introduction as well. Evil Clown, Paladin, Vampire (yeah I know he isn't actually, vlad fans, but c'mon don't kid yourselves), Tiny Spellcaster (x2), etc.
Urgot and Yorick of course deserve honorable mentions as champions that are no longer allowed to exist in a competitive state due to their kits. And Evelynn, while not having a strictly bad kit, completely failed to end up as an assassin as intended. This is all old news though.
As a small personal grief, Nidalee jungle bothers me. Not really anything to do with her being in the jungle role, which has maybe been the best place for her to exist without being unmitigated amounts of frustration to face in lane. However, the amount of her performance which is locked behind arcane PvE mastery feels like a regression back to dark ages LoL jungling.
I'm going to say Ekko comes high on the list for me in terms of mechanics. Pretty much a combination of the most frustrating to deal with mechanics in one champion with the added bonus of being a forest fire of balance concerns.
's escape,
level regeneration , and everything that feels kind of bullshit to deal with but you could get over if that was the only deal with a champion's kit. These were the ingredients chosen to create the perfect little assassin. But Riot accidentally added an extra ingredient to the concotion...The ability to build tanky!
Thus,
was born! Using his ultra-bullshit powers, Ekko has dedicated his life to fighting fun, and the forces of balance!