Kha'zix and Evelynn.
If you want to give a champion the ability to Stealth, fine. If you want them to be able to one-shot almost instantaneously, I guess so be it. But if you also want to give them escapes or resets to get out of the fray once they've entered, that's where the bullshit happens. Say what you want about Twitch (I hate him too), but at least when he shows to attempt to delete your entire team with 3-5 autos and E he is revealed and doesn't have any escapes outside of Flash. Kha'zix can stealth through every damn brush, delete a carry, then hop out towards and bush and he's gone. Bullshit. Evelynn can walk around stealthed, pick off a carry (or honestly even a bruiser her damage is so stupid), and R out to safety. Her damage is so high that sometimes she doesn't even need to use her R for the execute. The combination of Steath, One-shot potential, and escapes is absolutely toxic providing very little (if any) counterplay.