Sentinel is pretty high up there. Its mostly because of him that a lot of the lower tier characters aren’t playable in MvC2 since his keep away is so strong, in addition to his massive damage with everything he does and his speed. Cable as well. Magneto and Storm are a bit more manageable since they can’t chip you to death as easily, but Storm is still massively frustrating to play against with a low tier unless you’re packing a godly AAA like Cammy or Capcom. But against Sentinel, even with those assists you’re not getting anywhere if you’re using someone that doesn’t have some sort of extra air mobility. MvC2 still manages to be a good game despite this, but I personally think that’s because everyone that’s good would be broken in any other fighting game, and it just happens to balance itself out in the end.
Hilde is pretty darn broken in SCIV. She hits you with her level 3 B, you’re taking half your health in damage and being carried half way across the screen to a potential ring out.
The top 3 in BB:CT are quite strong, but I don’t think they quite manage to reach broken tier. Nu deserves special mention though because of how blindingly simple it is to do decently with her. Rachel and Arakune are at least somewhat difficult to use.
These characters are actually banned:
There’s Akuma from SSFIIT. Can’t be dizzied, air fireballs, infinite block string with red fireball in corner, and probably some more stuff I don’t know.
There are several bosses that are probably broken tier along with Akuma, like Gill (SFIII) as well as Justice and Kliff (Guilty Gear series).
Archetype Earth from Melty Blood: Actress Again has an attack that OHKOs the entire cast (except one character for storyline reasons), has a special super meter that is constantly regenerating meter, has like 5x the damage output of everyone else naturally (including off of a throw), extremely fast, and extremely good range.