Slowking, the counterpart to Slowbro is the less popular of the Slow-twins as evidenced by its lack of a Mega Evolution in later generations. Furthermore, bulky Water types prefer high DEF to SDEF as many of their targets (Rock, Ground and in the Slow-twin's case, Fighting) are largely physically oriented. Not that Slowking doesn't have some merit of its own. That high SDEF allows Slowking to take on massive special threats that would just cleave through Slowbro, and as a psychic type it has a wide array of support moves as well as a moveset that many pokémon would love to have in how varied it is.
Where's My Pants?
Ability: Regenerator Item: Leftovers/Heavy Duty Boot
-Teleport Nature: Bold (+DEF, -ATK) OR Calm (+SDEF, -ATK) EVs: 252 HP/252 DEF/4 SDEF OR 252 HP/252 SDEF/4 DEF
Standard Bulky water set. Scald is amazing on Slowking as Burn makes Slowking immensly bulky on both sides of the equation and almost everything immune to Burns does not want to be eating a Scald anyways. Slack Off gives Slowking reliable recovery, enabling it to take repeated hits over the course of the match. Future Sight and Teleport combo well together, enabling Slowking to safely pivot out into a counter and then chip the counter's counter when Future Sight goes off which can be huge in starting a sweep.
The King's Domain
Ability: Oblivious Item: Sitrus Berry/Colbur Berry/Iappa Berry
-Imprison Nature: Relaxed (+DEF, -SPE) OR Sassy (+SDEF, -SPE) EVs: 252 HP/252 DEF/4 SDEF OR 252 HP/252 SDEF/4 DEF
The Slow-Twins have a unique leg up on most Trick Room setters in that they have access to the ability Oblivious, meaning your opponent HAS to beat them down and cannot shutdown Trick Room with Taunt. Scald and Muddy Water are for STAB (reliable single target with Burn VS Spread), Protect is near mandatory in doubles and helpful for Slowking who will likely get double targeted in an attempt to avoid Trick Room all together. Imprison is the last move of choice, and is amazing in sealing off your opponents Protect (so they can't stall out Trick Room) and opposing Trick Room users (in the case of a Mirror match or as counter play to Trick Room).
Item is to help Slowking survive the double target by giving it some immediate healing/to stave off a Super Effective blow
Yawn -Guaranteed Sleep that will often force your opponent out? Yes and Yes. Thunder Wave -Because Speed Control is always fantastic and no Ground type (except Gastrodon and Seismotoad will willingly come in on Slowking) Icy Wind/Bulldoze -Two other double staple moves though Slowking is a bit slow to be using them. (more speed control) (doubles only) Heal Pulse -When you need to keep your partner alive no matter what (doubles only) Iron Defense/Calm Mind/Stored Power -Gimmicky set where you have Slowking buff itself to be unkillable and then nuke the opposing team with Stored Power (singles only) Nasty Plot -Slowking's Special movepool is amazing and works well with Nasty Plot, but Slowking is rather slow be attempting a sweep. Foul Play -Cause Slowking attracts physical attackers and special walls, this works nicely in catching them off guard.
Galarian Slowking Ability: Regenerator Item: Leftovers (or anything to improve bulk)
Bold or Calm Nature EV’s: 252 HP/252 SATK/4 DEF or SpDEF
Due to having slightly higher Special Attack and access to Hex, Galarian Slowking could run a Hex set. The primary method of inflicting status is Yawn or Thunder Wave, with the latter being an option since Yawn often results in a switch. The other two moves are STAB options, or Scald if desired.
Eerie Spell may be worth mentioning too as an STAB option. The lower PP is a shame, but draining the opponent's PP in the process can serve a niche role against certain threats. Also it's a sound based move, so Substitutes can be bypassed. But yeah...I guess Psychic and Psyshock would be more reliable...