Stakataka -> B
Stakataka seems underranked at the moment, and I feel that it's really unexplored. A physical attacker that can 2hko lando-t AND skarmory is already impressive, but pair that with a spammable stab move named gyro ball that heavily dents even resisted pokemon like excadrill, kartana, and zapdos, and you've got a wallbreaking powerhouse. This allows ease of prediction, unlike other wall breakers like tyranitar, kartana, and mega-medicham. This means that under trick room, it has the capability of getting 1-2 ko's off if the opponent has a good check to stakataka. The only pokemon in the entire tier that doesn't get 2hko'd by gyro ball + stab move is physically defensive tangrowth, which has a high chance to get 2hko'd by gyro ball after rocks, rotom wash and skarmory, which both get 2hko'd by stone edge, and ferrothorn, which can be taken down with superpower if the team opts to run it. The only counter to stakataka that I can think of off the top of my head is mega slowbro, which is frankly very bad in the overused metagame. With its immense wall breaking power and its access to trick room, stakataka will be able to hold its own in every match. Here are some calcs to illustrate the power of stakataka.
-1 252+ Atk Life Orb Stakataka Gyro Ball (150 BP) vs. 252 HP / 216+ Def Landorus-Therian: 187-222 (48.9 - 58.1%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
-1 252+ Atk Life Orb Stakataka Gyro Ball (150 BP) vs. 0 HP / 4 Def Landorus-Therian: 255-302 (79.9 - 94.6%) -- 50% chance to OHKO after Stealth Rock
252+ Atk Life Orb Stakataka Gyro Ball (150 BP) vs. 0 HP / 0 Def Greninja: 244-289 (85.6 - 101.4%) -- 87.5% chance to OHKO after Stealth Rock
252+ Atk Life Orb Stakataka Gyro Ball (150 BP) vs. 200 HP / 0 Def Magearna: 313-370 (89.1 - 105.4%) -- 68.8% chance to OHKO after Stealth Rock
252+ Atk Life Orb Stakataka Gyro Ball (150 BP) vs. 0 HP / 4 Def Kartana: 139-164 (53.6 - 63.3%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Life Orb Stakataka Gyro Ball (150 BP) vs. 0 HP / 0 Def Keldeo: 192-226 (59.4 - 69.9%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Life Orb Stakataka Earthquake vs. 252 HP / 0 Def Toxapex: 221-260 (72.6 - 85.5%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Life Orb Stakataka Stone Edge vs. 248 HP / 252+ Def Skarmory: 134-160 (40.2 - 48%) -- 55.1% chance to 2HKO after Stealth Rock
The set I was using consists of Gyro ball, Trick room, Earthquake, and Stone edge, but there are other viable sets too. Lonely with 14 defense IVs allows beast boost to boost attack upon fainting a pokemon, which makes it much harder to revenge kill. Choice band is a set that forgoes trick room and opts for more sustain and power at the cost of locking stakataka into one move. Rockium Z allows stakataka to beat rotom-w, skarmory, and celesteela more consistently at the cost of life orb.
As with every pokemon, Stakataka does have its downsides. It's mainly outclassed by banded ttar as a wall breaker, who also pressures stall and can pursuit trap. It also requires trick room support, which although it can provide itself, wastes a turn and also requires that the team dedicates several or all of their pokemon to trick room abusers. It also suffers a weakness to specs ash greninja's water shuriken, and that its defense stat is just low enough for defensive lando-t's eq to ohko. The rock/steel typing is great offensively, but it's a double edged sword which really restricts the amount of times stakataka gets to set up trick rooms and set up shop.