Muk

Muk is a very viable Pokemon in the PU metagame due to its high Attack stat, good special bulk, and pure Poison typing. It's blessed with two fantastic STAB moves in Gunk Shot and Poison Jab as well as access to the elemental punches and even a priority move with Shadow Sneak. This combination of a fairly wide movepool and its well-placed stats allows it to function well as a wallbreaker, special wall, late-game sweeper, or just a simple tank. Muk also happens to be a great check to some of the PU metagame's most prominent Pokemon, including Rotom-F, Cacturne, and Clefairy. Unfortunately for Muk, Poison is not the greatest offensive typing, as it is resisted by the very common Ground- and Rock-types, and Steel-types are immune to it. This requires anyone using Muk to predict more often than they might with other Pokemon, oftentimes forcing it to rely on its much weaker coverage moves in order to deal with its switch-ins. In addition, its low Speed stat lets it down considerably; Shadow Sneak may make up for this to an extent, but it doesn't do enough damage to revenge kill much outside of physically frail Pokemon that are weak to Ghost and Pokemon that are already weakened. Lastly, Muk lacks any form of reliable recovery, meaning it must rely on Rest or the support of its teammates if it wants to survive in longer battles.