Gill (3s) is broken, but he is a boss character. As a regular character, probably Ciel (in mbac incarnation) have numerous advantages in mechanics (dash with clash frames , a ex-edge called hiero that can be used to punish anything because it have a quick startup and the hitbox is simply all the screen, just to say some advantages…).
I’d wager that Superman in Justice League Task force is more broken:
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Can we say Mugen? Then Shin Tom Hanks. But at least he’s funny. Check out on youtube if you don’t know him. XD
Doesnt count.
Doesnt count.
Pojo the chicken, from Mace the Dark Ages.
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Loved that fighting game, even though the end boss was insanely difficult to beat…then you find out Pojo is pretty much unstoppable. If you get hit, its because you either you fucked up, had a brain hemmorage, dropped your controller, or were abducted by aliens.
Honorable mention needs to go to Dong Dong Never Die. Game has a character that has a projectile infinite from across the screen and a guy who can cancel one of his specials into the same special. AMAZING!
Nigga Dick Jax
The boss character from that Arcana Heart port was also pretty damn broken if I remember correctly.
Also Archetype Earth in Melty Blood AA. She’s intentionally broken and banned from tournament play.
Toki from Hokuto no Ken Fighting !
Arcade/PS2 !
He’s not even the best character in that game.
Hahahaha WHAT.
What? Who is better than Toki?!
Rei 
This.
I hope that was a joke.
It’s not. Rei is better than Toki, though only by a slim margin really.
I know it’s a boss character, but I just wanted to mention Karai from TMNT: Tournament Fighters (SNES). She’s selectable with a code (and so is Rat King but he’s not quite at her level).
That bird they banned in JoJo’s. The whole cast had like a 9-1 matchup vs him. He was surprisingly similar to Ivan Ooze.
-GGX SLASH KY
-hnk toki
-BBCT nu
-cedric from battle fantasia
-jin form T4
-the sisters from sc3