I'd agree with Thundurus to S rank - definitely one of the most versatile and deadly pokemon in the metagame.
Some other nominations:
Aerodactyl-Mega B ---> B+
Nice anti-meta pokemon: Outspeeds and OHKOs a lot of big threats (Thundurus, Landorus, Keldeo, Gengar, Charizard). Can give stall trouble with Roost + Taunt. Does not get recoil-less Head Smash and Brave Bird like regular Aerodactyl, but its extra bulk lets it tank priority better and Tough Claws boosted Brave Bird still hits hard.
Azumarill A ---> A+
Probably the best water type in a meta where the top ranked pokemon are almost all weak to Water. Azumarill has a lot of great options available to it - including two great priority moves in the form of ExtremeSpeed and Water Shuriken. It can run all out attacking sets with an Assault Vest or Choice Band, it can go down the set up route with Shell Smash or Belly Drum, or can even work well in a more defensive role thanks to its unique typing.
Chesnaught Unranked ---> C
A decent check to some of the metas biggest threats (Diggersby, Tyranitar, Terrakion). Leech Seed + Spiky Shield + Drain Punch/Horn Leech = recovery for days. Chesnaught is a pretty good Spikes stacker too. However, a quad weakness to Flying in a metagame full of genies is not ideal.
Diancie C ---> B-
Despite its many weaknesses, Diancie's typing can be really useful. It has a niche as a Geomancy user that resists ExtremeSpeed - making revenge killing it much harder. Its typing also allows to check to Thundurus, Talonflame, and Tyranitar (providing it doesn't have Iron Head or something) in one team slot.
Dugtrio B- ---> C
Unless your team really needs to trap Heatran, there are probably better choices than Dugtrio. I guess it does have the niche of being able to trap things and set up Spikes or Memento. But it is super frail and not very strong, in a meta full of priority.
Empoleon B+ ---> C+
Why exactly is Empoleon B+? Scald and King's Shield is a nice combination and Defog/Stealth Rock support is nice, but it seems outclassed in most of what it does.
Ferrothorn B ---> A
Ferrothorn is really great in the current metagame. King's Shield + Leech Seed is really useful for scouting/recovery and it can check things like Azumarill, Diggersby, Tyranitar and Gyarados.
Forretress A ---> B
Having access to every kind of entry hazard while also being able to Rapid Spin is pretty good, but I rarely see Forretress getting chance to do either. It is total Taunt/Set up bait.
Gengar C ---> B+
Probably the best Diggersby counter for offensive teams and one that has plenty of utility outside of countering Diggersby. The Substitute/Taunt/Will-o-Wisp is a nice stallbreaker. Can easily set up Toxic Spikes on the switches that it forces.
Gliscor B ---> A
As many OML players are seeing, Gliscor is fantastic in the current metagame. Its status immunity gives stall a hard time and makes Sableye a non-issue. Its typing makes it a good check to Tyranitar, Diggersby, Lando-T and Terrakion. Gliscor has a lot of great support moves like Taunt, Spikes, Defog, Stealth Rock and Tailwind. It can even run all out offensive sets since Poison Heal mitigates the recoil from Brave Bird.
Gyarados-Mega Unranked ---> B
Mold Breaker allows it to get away with running a mono-attacking set-up set and not care about Water Absorb or Unaware - making Substitute + Dragon Dance + Roost + Waterfall a deadly combo. The fact that most of the top ranked pokemon are weak to Water works nicely in Gyarados' favour.
Klefki D ---> C-
I've seen OML players using a Geomancy set which was surprisingly not terrible - being able to Sub vs Sableye before it can Topsy Turvy you seems kind of fun as well. Klefki gains some nice moves like King's Shield and Moonlight too, as well as being able to do its usual Spikes/Screens/Prankster Thunder Wave shenanigans.
Medicham-Mega Unranked ---> B-
Mega Medicham remains a great wallbreaker in STABmons. No longer having to rely on HJK is useful since you no longer have to worry about losing half your health to random Protects or Ghost type switch-ins. Mach Punch gives it powerful STAB priority against more offensive teams. It's still kinda frail though so opposing priority is an issue.
Porygon2 D ---> B-
Porygon2 is actually pretty good. A nice tank which can check a lot of things depending on which defense you decide to invest in. Glare is great for paralysing any non-Electric types and Boomburst hits hard even without special attack investment. Trace and Download are both great abilities. It gets a lot of nice support options too, including Whirlwind, Heal Bell, Super Fang, Baton Pass (works well with Download), Toxic, Rapid Spin, Trick Room, and Lovely Kiss.
Shedinja Unranked ---> C-
Almost all of Shedinja's weaknesses are commonly seen attacking types in STABmons (Flying and Dark especially), but it can take advantage of the stuff it does wall (most Keldeo, Diggersby and Meloetta for example) by setting up Spikes or Sticky Web on the switches it causes. It definitely requires a lot of support, but it can be a decent suicide lead.
Tangrowth C- ---> B-
Another pokemon that has shown its true potential in OML teams. A nice check to things like Diggersby, Tyranitar and Azumarill. Has a lot of nice support options like Leech Seed, Aromatherapy, and Spore.
Terrakion B ---> A-
Fighting/Rock is amazing STAB coverage to have in the current metagame (hitting more than half of the S and A+ ranked pokemon super effectively). Terrakion also provides a valuable normal resist. One of the best Stealth Rockers for offensive teams, and the Swords Dance and Choice Band sets are great wallbreakers.
