Personally to me..balance can kinda be overrated
take Marvel vs Capcom 2...a game that is BEYOND terribly balanced and is still recognized as one of the better fighters. Same with Smash Melee and don't even get me started on Pokemon.
So whether it be a single player RPG or a multiplayer Deathmatch what's some of your favorite OP ..well anything
Persona 5: Yoshitsune...just the fact you can make a build that's immune to everything and does a crap ton of damage is just amazing....the fact it takes a bit of planning and work to get him is the icing on the cake
FF7: Powersoul...holy crap its amazing how obscene this weapon is....Death Sentence, Low Life Tifa just destroys the entire game...
FF6: Vanish+Doom. FF6 got LOTS of janky OP abilities....this is just one of them Merit Badge Gau is another great example