Might get shamed for this
Nomination:
Delphox
A->A+/S
Okay so going through the viability rankings and tier itself, Delphox has no true counters bar Houndoom (and Mega Audino if you'd really wanna use your mega slot on that). Anything that could possibly switch in to its STABs can take massive damage from a coverage move that it usually runs. While not the highest in the tier, 114 SpA and 104 Spe is pretty solid stats for RU. Offensive checks are plenty but the switch in game doesn't really exist. Between Fire Blast, Psychic/Psyshock, Grass Knot, and Dazzling Gleam it can hit potential counters like Toad, Scrafty and Gastrodon super effectively. By the definition of a counter, it takes on the same role the Kyurem-B and Hoopa have in OU, with potential coverage for nothing to
safely counter.
Next idea, style-breaking.
Slap a scarf on it, and it can easily dismantle offensive teams, outspeeding most things but of course missing out on a couple OHKOs and stuff. Still, wearing down an offense team makes it a horrifying cleaner when whatever defensive options are worn down. Outspeeding Flygon and Hitmontop is also a great revenge killing option with Psychic and/ or Dazzling Gleam and is a nice option for hazard stacking teams. Hazard Stack + Delphox (haven't really tried it) sounds like a nightmare for offense in general.
Next, Life Orb or Choice Specs boosting 114 SpAtk, the 6th highest in the entire tier (bc Mega Glalie's 120 doesn't matter), can allow it to dismantle defensive cores with its coverage. Hell, Switcheroo can neuter a fat mon that it usually couldn't handle. Plus, even unboosted it sits at base 104 Spe, making it more useful in matchups vs fast offenses unlike Mega Cam or Clawitzer. Even SubCM sets can surprise and damage defensive cores by forcing attacks.
So basically what I'm saying is that you can mold Delphox to match up well vs Offense or Balance, whichever your team needs. Might expand later, short on time rn