Muk: THE tank. With high natural bulk, usable offensive and defensive typing, good Attack, and low Speed, Muk is the pure definition of a tank in the RU metagame. While it's outclassed as an offensive tank on balanced and offensive teams, Muk does have a niche on full stall teams by combining a bunch of various roles into one Pokemon. Muk's unique traits allow it to perform as a Trick and Toxic Spikes absorber, Toxic spreader, Rotom-C counter, and Durant, Escavalier, and Qwilfish lure all in one Pokemon. Muk is a formidable specially defensive tank, but its Defense leaves a lot to be desired and it's easy enough to take out due to its lack of reliable recovery. In most situations, Qwilfish is a better Poison-type for defensive teams thanks to Intimidate and its access to Spikes, Toxic Spikes, and Thunder Wave, but if the roles described before are needed in one teamslot, Muk is your Pokemon.
This set should only be used on full stall teams, as Muk will lose too much momentum for the team otherwise, and it doesn't hit quite hard enough or fulfill a role useful enough on balanced and offensive teams to warrant a teamslot. Muk's foremost role on the team is as a Grass-type counter, particularly against Rotom-C, which can otherwise give stall teams some trouble, and the moveset helps accomplish this. Poison Jab has good neutral coverage in RU, the poison chance is nice, and it absolutely demolishes Rotom-C and lands a good hit on other Grass-types as well. However, it should only be used if the Pokemon you predict to switch in isn't more harmed by Toxic. Fire Punch puts the hurt on Amoonguss and Roselia, but it's more useful for luring in and KOing Durant, Escavalier, and Ferroseed, which are all very dangerous Pokemon to give a free turn to. Thunderbolt's main role is to 2HKO Qwilfish after Stealth Rock damage, and it gets a decent hit on Moltres and other Flying-types. The last slot will usually go to Toxic, which deters a lot of the Pokemon that will generally be switching into Muk such as Slowking, Mesprit, Rhydon, Quagsire, and Gallade. However, Haze is usually a requirement on stall teams, and if you're not using Cryogonal or another Pokemon for the role, then Muk fits the bill well. Unfortunately, most of the Pokemon that Haze is most useful against can threaten Muk with their STAB moves or OHKO it if it switches in against the likes of SubCM Uxie, Mesprit, Sigilyph, Gallade, and Kabutops.Team Options & Additional Comments >>>
Muk's movepool is huge and thus has a ton of other options and sets it can utilize in RU, though none that really offer a whole lot over most other RU Pokemon. Brick Break, Hidden Power Water, or Hidden Power Ground could be used on the specially defensive set as they're Muk's best weapons versus Aggron, Steelix, and Rhydon. However, they're generally not worth the moveslot unless your team can secure Toxic Spikes on the opponent's side of the field, thus freeing up Muk's last moveslot in place of Toxic. An offensive set with full Attack investment and an Adamant nature with Poison Touch + Poison Jab can be incredibly frustrating for the opponent, as the poison chance is very high. On more balanced teams, this spread of poison and consistent damage can provide the openings needed to secure a late-game sweep by something like Dragon Dance Fraxure or Swords Dance Feraligatr.
More classic offensive Muk sets include a Choice Band set featuring Gunk Shot and a SubPunch set. Both offer a lot of power backed by Muk's good defenses, though Muk is just so slow and the metagame so hostile to it that it'll usually end up being dead weight. There are also just more reliable wallbreakers available in Pokemon like Medicham, Gallade, and Druddigon, so it's rarely worth giving up a teamslot for Muk in place of them. Other more obscure choices include Curse, Memento, Taunt, and Torment, but Muk's typing is just not suited for creating a set around any of those moves, as it'll usually just be KOed before they can be effective.
Checks and Counters
Steelix and Aggron just laugh at anything Muk can throw at them apart from the odd Hidden Power Ground or Water on the main set. Choice Band Brick Break can hurt Aggron, but that's pretty rare. Other Steel-types are immune to Poison Jab and Toxic, but switching into Fire Punch can mean trouble, particularly for Escavalier and Durant. Earthquake is the easiest general way to eliminate Muk from the scene, and there are plenty of users of it in RU including Rhydon, Crustle, Druddigon, Aerodactyl, Archeops, and Drapion, the last of which even being immune to Toxic. All of these Pokemon are also good general answers to more offensive Muk sets.
Clefable and Sigilyph are annihilated by Gunk Shot, but both make great counters to the main specially defensive set, as Poison Jab and Thunderbolt simply don't do enough, and they're immune to Toxic to boot. Psychic-types such as Uxie, Gallade, Mesprit, and Medicham generally have an easy time with Muk, particularly if they aim for its lower Defense stat and avoid Toxic (though Uxie with Heal Bell doesn't even have to care about that either). Muk is most effective if the opponent doesn't have recovery, so sending in something with recovery will generally force Muk out, especially if it can't poison it. Muk's lack of recovery outside of Black Sludge means most of the damage inflicted on it is permanent, and its weakness to entry hazards means it'll get worn out quickly over time. However, be aware that Muk is generally paired with Wish users, so oftentimes it'll mean not allowing them to get a Wish pass off to Muk in order to defeat it.