Slowking has seen some play in VGC over the years as a reliable Trick Room setter thanks it combination of its bulk and access to Oblivious preventing Taunt from denying Trick Room. Setting Trick Room isn't Slowking's only trick, with wide coverage and decent Special Attack, Slowking is a decent Trick Room attacker, while Yawn is great at forcing opponents out of position. Sadly the passing generations have been brutal for Slowking. The introduction of Hatterene has all but superseded Slowking as a Taunt immune Trick Room setter, thanks to its wholly superior Magic Bounce blocking not just Taunt, but other forms of disruption like Spore, Encore, Toxic and Thunder Wave while boasting far more potent Special Attack to sweep under Trick Room with. Even then, Taunt is infrequent, with the most common methods of dealing with Trick Room are Fake Out and focus firing, which Slowking is ill equipped to handle, as opposed to other Trick Room setters like Porygon2, Farigiraf and Indeedee-F alongside more niche options such as Oranguru, Gothitelle, Cresselia, and Gallade, all of whom are more reliable in setting Trick Room. As for all of Slowking's prodigious bulk, its typing leaves to a lot to be desired, leaving Slowking exposed to many staples of the format, including but not limited to Incineroar, Rillaboom, Gholdengo, Ogerpon, Sinistcha, Archaludon, and more. Galarian Slowking, while having more oomph behind its blows and a not as vulnerable typing suffers from much the same. Even when looking beyond Oblivious to Regenerator, Slowking just becomes at best a shoddy knockoff Amoonguss. The Curious Medicine of the Galarian form is cute, being a self haze for Slowking's allies, but physical attackers are generally better either running Clear Amulet, and Galarian-Slowking's playstyle doesn't match with most Draco Meteor users bar Hydrapple. The fact that switching out also wipes stat drops, the same thing you are doing to get Galarian Slowking in and Curious Medicine becomes rather suspect.
In Singles, this is a very different story, with Slowking being bar none one of the best pivots in the game. While the loss of Teleport is unfortunate, Slowking's sluggish speed all but ensures its ally is getting in unscathed if it has any pivot move and Gamefreak was more than kind to give Slowking a signature pivot move in Chilly Reception, which also has niche effect in causing weather disruption in setting Snow. Slowking is just built to pivot, with Regenerator recovering health every time it switches and Future Sight applying pressure even after Slowking switches out. With this, Slowking becomes the glue for a team, the pivot point to accurately answer and maintain positioning no matter what your opponent throws at you.
Dogged Comedian
Slowking-Johto
-Chilly Reception
-Future Sight
-Scald
-Slack Off
Ability: Regenerator
Item Attached: Heavy-Duty Boots
EVs and Nature:
252 HP / 252 Def / 4 SDef
Relaxed Nature
Tera Types: Water/Fairy
Pivot Slowking is amazing as mentioned before. Chilly Reception with Slowking's molasses speed ensures it is pivoting after the opponent moves, effectively giving any Pokémon a free switch. Future Sight pairs wonderfully with this pivot, putting further pressure on a switch as few Pokémon can with stand the one-two punch of Future Sight (a base 120 STAB attack mind you) and whatever damage Slowking partner applies, which is great at generating momentum and reaching KOs not possible. Scald prevents Slowking from being entirely passive and shores up its poorer defense thanks to its great Burn chance, something Slowking's allies can also capitalize on. Slack Off rounds out the set, but is important for recovering damage that Regenerator doesn't, keeping Slowking around for longer to continue pivoting.
Preferred Partners
Bug, Fighting and Fairy types are strong recommendations for Future Sight Slowking, as they directly threaten the Dark types that can otherwise no sell Slowking's Future sight, placing the opponent between a rock and a hard place, as well as being a common pivot option into the Dark types looking to directly remove Slowking. Lokix in particular loves Slowking's slow pivot, its extreme fragility meaning free switches are rare for it, while its plentiful priority options helps cover Slowking against faster more agressive teams. Weavile too excels similarly with Slowking, even getting a nice Defense buff from Chilly Reception in the process. Scizor is also notable, its typing making it adept at handling Slowking's Grass weakness as well, while providing a pivot option into Slowking with U-Turn, dodging the Fire types that would be used to dispatch it.
Other Options
Thunder Wave is a powerful status option that makes Slowking's pivots all the more damaging by ensuring whatever comes in is going to be faster and apply that much more pressure, though it does exasperate Slowking's Electric weakness.
Psychic Noise is niche, but a good option against Stall teams where Slowking is less impactful against by denying their healing for a time.