I'd say Shedinja, because while it isn't overcentralizing nor is it exactly broken, it's ruinously uncompetitive. If everyone used it, everyone would pack coverage for it because it required that, and as such it wouldn't be a problem - but currently it lies in that lowish usage where that's not exactly necessary and so it can be an insta-win against some teams.
And of course it's not just Shedinja I'm talking about, who is rocks weak, but also the pairing with Mega Sableye who keeps those rocks away (who I fully agree should remain OU, to not open that can of worms). Not every team wants to be running sand or something like SR Clefable to deal with the pair, and as such it's very unfair for players who are punished for wanting to create a certain archetype of a team that doesn't deal with that niche rubbish.
For an example I have an offensive Mega Gardevoir on my team right now, and I could run Shadow Ball in the place of Focus Blast for such occasions (and to hit Mega Metagross, Jirachi or opposing Gardevoir hard who switch in) - but is that good coverage against steels (like the vastly more common Heatran) or other pokemon in general? No it is not, and so I refuse to run it. Players who utilise the core also normally include the OU non-banned trappers, so that what threats there is to the core they can attempt to weed out and eliminate.
These teams win because people like me refuse to co-opt in running coverage for them, and so they have an arbitrarily higher rate of success. That is what makes them uncompetitive as well as no fun and no good for the ladder. People seemed to agree that Gothitelle wasn't exactly OP, and yet it was banned because it was uncompetitive and that sucked. Whilst Shedinja doesn't see the same usage, that still doesn't stop it from sucking just as hard if not harder.