Shaco, probably. His kit has too many powerful tools in it (a precursor to irelia and jayce, really), and in general is just designed to be a dbag all the time. Combined with the whole 'terrifying early game, crippled late (teamfight) game, he just seems like he was terrible design'

Jayce. Not OP but his kit is way over the top. Pretty much the only thing he's missing is a stun.

Graves.

Yeah, I said it. What's so special about crappy nuke, generic dash, mini-Nocturne ult, and bigger nuke? His lore is pretty substandard too, which isn't a big deal on its own, but paired with how poorly designed his kit is, he's a big mediocrity sandwich.

Jayce, hands down

Skarner was a cool idea who is 50% finished. The movement creep he can't keep up with, ult redo, and lack of E is the other 50%.

Worst design: Katarina ranks as one of them imo. Sorakas pretty bad offender too. And shaco because he is pretty much antifun for everyone but the actual shaco player.

Sion. Regardless of how you build him two of his skills will be useless.

I think it's a six-way tie between Fizz, Lulu, Nautilus, Orianna, Renekton, and Riven.