If I were to honestly answer this without bias, I'd have to go with two champs.
Yorick - He has amazing sustain, harass, damage, and his ability to not die, or to bring back the dead adc or apc in the middle of the team fight is fantastic.. His early game is amazing, his mid game more-so, and his late game is pretty much that of a carry, except with a different job in mind. His power level at all points in the game is strong.
New Twitch - Not going to lie, this might be biased, but when I picked up Twitch after his remake, his ability to disrupt the team fight without cc, his ability to de-focus himself, his range/dmg while his ult is active, makes Twitch a Pentakiller. To do it right, you need the right build, obviously, but mostly you need positioning. n the right hands, I feel Twitch is possibly the strongest adc in the game atm.