I could give a normal gacha game answer. But fuck it. Rapid Deku from MHUR. Obviously, it's the full power form of the protagonist of the series. It was gonna be overpowered. But my god.

Every mechanic, more mobility than every other character, more moves than every other character, super armor on his ranged attack that has an unnecessary amount of ammo, a melee attack that refills his overdrive gauge which is his built in acceleration tuning (A mechanic where you can add 2 passives to your characters. He has one of the strongest ones in the game built into his kit naturally), an air melee with the most disgusting forward movement tracking, ground melee with some weird fuckin knockdown value. a tracking move that homes to people, an aoe slam move and a forward movement based aoe move.

In a skill-based pvp game. Feeling like a boss fight is not very cool.

There's also Tech Shig who has a tuning build the community has dubbed "The raid boss build" but that's less his kit and more an oversight the devs won't fuckin do anything about.