I’m going to say Mercy. Damage boost skews the balance of equal number of characters into something like 5v4 but the 4 have 2 tanks. Res is also a get out of jail free card a lot of the time.
Mercy
Moira, illari, heck all dps"supports", all dps"tanks".
Lifeweaver. His lifegrip specifically is by far the worst ability they added to this game. A bad lifeweaver (even if he means well) can get you killed in multiple ways or just throw fights entirely because of bad pulls.
That being said, there are a lot of heroes that are poorly designed right now such as Mercy, Hanzo, Illiari, … but their numbers can be tweaked in multiple ways for most part to balance things.
With lifegrip you can’t really do that as lifegrip’s problem isn’t the numbers, it’s how the ability works.
Honestly every character added after Ana.
Sombra and it’s not even close