Either Doomfish, or Sombra.
Doomfish just feels like an out-of-control monster whenever I encounter him, and I feel like his kit is a little too versatile, mobile, and oppressive. Especially since I primarily encounter him in Mayhem (where he’s the most broken character in a broken mode) or in FFA (where he’s just always in your face). Furthermore, he just has an awful personality, so when he smashes you and growls “Stay down” or whatever it feels like being at the receiving end of a bully.
Sombra, on the other hand, exists only to ruin fun and negate characters. She has a useful function, but is still one of the most frustrating characters to deal with in general. Hypocritically, I like having her on my side, especially if she can shut down the Doomfist on the other team. I’d rather we didn’t need her at all… or that she’d leave me alone.