I'd like to nominate a pokemon that probably seems completely unviable, but I think it has unique traits that make it distinguishable and viable in the metagame. This mon can be used as a wall as well as, at times, a wallbreaker.
UR --> C-
I've been using this set:
Slowking @ Assault Vest
Ability: Regenerator
EVs: 252 HP / 252 SpA
Modest Nature
- Scald
- Ice Beam
- Fire Blast
- Psychic/Psyshock/Future Sight
Ice Beam takes out Landorus, Zygarde, and other sweepers. Fire Blast pins defensive steel types like Celesteela and Ferrothorn. Psychic takes out Hawlucha and forces out Toxapex. Scald is meant to hit Heatran and general switch ins to fish out for burns.
The rest of this post is basically trying to justify this over Magearna and Amoongus.
It seems, at first glance that Amoonguss, although having less special bulk than Slowking, is a much better in basically every situation. It resists Koko and Greninja, two major special attackers in the metagame, while Slowking cannot. Plus, Amoonguss has access to Clear Smog, Spore, Toxic, etc., which Slowking does not/cannot have due to its need for AV. What separates Slowking from Amoonguss is that Slowking can hard counter Latios and most Heatran variants(excluding the basically nonexistent Bloom Doom variant) as well as act as a soft check to Lele and at times, even Greninja. More importantly, it has a higher special defense and diverse movepool. It’s higher special defense means that it doesn’t need to invest any of its EVs into special defense, and instead, transfer them to special attack. This, along with moves such as Ice Beam and Fire Blast, allow it to threaten walls/leads such as Landorus, Venusaur, Magearna, Ferrothorn(only with prediction/chip damage), and Toxapex, as well as sweepers/wallbreakers like Zygarde, Hawlucha, Blacephalon, and Latios. At the same time, Slowking can act as a wall towards many special attackers and even some physical attackers because of its respectable defense and Regenerator. Slowking can afford to stay in to many physical attackers an retaliate with powerful coverage moves. Amoonguss, although also able to take some physical hits when needed, does not have the coverage to justify it staying in to most physical attackers. Slowking, unlike Amoonguss, can pose a legitimate threat to offensively oriented teams, while also maintaining its position as a special wall.
Magearna also seems to be a wall that also outclasses Slowking. It has more special bulk, higher special attack, and a way to gain momentum. It can counter Koko, Greninja, and Lele much better than Slowking could. Its special attack allows it to pose a threat to many frail offensive pokemon as well. However, again, Slowking has a lot more useful coverage moves. It is able to counter many ground type pokemon, which Magearna simply cannot do. Magearna, without any healing ability or move, is left susceptible to chip damage, while its frailness physically prevented it from staying in to many physical attackers which Slowking can handle due to access to recovery and powerful moves. Slowking also has Future Sight, which can break momentum of the other team, which can somewhat make up for Slowking's lack of a pivot move. Overall, Slowking gives up the ability to check some important special attackers to being able to counter various physical attackers as well. Due to Slowking’s ability, it is able to scout, and can be played with more risks, while Magearna has to be played very carefully.
Overall, although Slowking can be seen as a less potent wall than other special walls prevalent in the OU metagame, its special bulk combined with offensive ability and Regenerator gives Slowking the ability to threaten physical walls and pokemon it even softly counters, giving it a niche in comparison to common special walls.
Some Calcs
252+ SpA Slowking Fire Blast vs. 252 HP / 208+ SpD Ferrothorn: 296-352 (84 - 100%) -- 6.3% chance to OHKO after Leftovers recovery
252+ SpA Slowking Ice Beam vs. 252 HP / 0 SpD Landorus-Therian: 432-512 (113 - 134%) -- guaranteed OHKO
252+ SpA Slowking Psychic vs. 248 HP / 252+ SpD Toxapex: 152-180 (50.1 - 59.4%) -- 74.6% chance to 2HKO after Black Sludge recovery
252+ SpA Slowking Psychic vs. 152 HP / 8 SpD Hawlucha: 390-458 (116.4 - 136.7%) -- guaranteed OHKO
252+ SpA Slowking Ice Beam vs. 188 HP / 0 SpD Zygarde: 376-444 (93 - 109.9%) -- 62.5% chance to OHKO after Leftovers recovery
Replays:
(Note: I'm also using Bayb's replays. I don't have a lot of replays showing Slowking countering physical attackers because I also use Slowbro-M.)
https://replay.pokemonshowdown.com/gen7ou-725623801
https://replay.pokemonshowdown.com/gen7ou-725179692
https://replay.pokemonshowdown.com/gen7ou-727322879