If Ekans is snake backwards, what is Muk?
Muk says "haaaaaayyyyy"

Muk says "haaaaaayyyyy"
Stats
Learnset (Courtesy of Bulbapedia)
Notable TMs/HMs (STAB in bold, most useful in italics, dubious underlined)
Breeding Moves (STAB in bold, most useful in italics, dubious underlined)
Useful moves from Last Gen
Overview: Muk, everybody’s favourite...thing, is back for another swing at competitive viability in Gen V. What’s still the same with Muk? Well, its stats, for a start, are excellently distributed for a defensive Pokemon, with 105/75/100 being quite potent, as well as a very nice base 105 Attack stat to abuse. Poison makes quite a good defensive type, resisting arguably the most common attacking type in the game in Fighting, along with Poison, Bug and Grass types. Quite a lot for a single type (ok Steel, whatever). On the offensive side, things are pretty grim. A Gamefreak exec must have drank Bleach as a child, because they have had it in for Poison types since Gen I. Only one type is weakened SE by it, and it is resisted by a pretty hefty 4 types (Poison, Ghost, Rock, Ground), and Steel types are even immune! This means poison types have to rely on auxiliary attacking types to do any damage, and all but loose that essential STAB bonus. But fear not, because Muk has these in spades, having a good physical movepool, and a few status options that allow him to own certain sets. Whats new? Depressingly, very little. Its primary ability in Stench got a minute boost with an added 10% flinch rate when hit by a physical move. It’s no Serene Grace, but still. It also gained a new ability in Underhanded, but seriously, just use Sticky Hold or Stench. Looking over its moves, its seems Muk gained most of his most useful moves last gen, with the elemental punches, Focus Punch and Shadow Sneak helping round of his type coverage.
So, how about some sets?
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So there we have it. Let me know if anything is amiss, or if I missed anything. Also, I suck at EVs, so if anyone wants to suggest better spreads, go right ahead.
HP:105
Attack:105
Defense:75
Special Attack:65
Special Defense:100
Speed:50
BST: 500
Attack:105
Defense:75
Special Attack:65
Special Defense:100
Speed:50
BST: 500
Learnset (Courtesy of Bulbapedia)

Notable TMs/HMs (STAB in bold, most useful in italics, dubious underlined)
Return/ Frustration
Brick Break
Venom Shock
Slime Wave
Flamethrower
Sludge Bomb
Torment
Rest
Fling
Explosion
Payback
Poison Jab
Rock Slide
Substitute
Brick Break
Venom Shock
Slime Wave
Flamethrower
Sludge Bomb
Torment
Rest
Fling
Explosion
Payback
Poison Jab
Rock Slide
Substitute
Breeding Moves (STAB in bold, most useful in italics, dubious underlined)
Acid Bomb
Curse
Haze
Imprison
Lick
Mean Look
Scary Face
Shadow Punch
Shadow Sneak
Stockpile
Spit Up
Swallow
Curse
Haze
Imprison
Lick
Mean Look
Scary Face
Shadow Punch
Shadow Sneak
Stockpile
Spit Up
Swallow
Useful moves from Last Gen
Acid Armor
Focus Punch
Sleep Talk
Fire Punch
Ice Punch
Thunder Punch
Focus Punch
Sleep Talk
Fire Punch
Ice Punch
Thunder Punch
Overview: Muk, everybody’s favourite...thing, is back for another swing at competitive viability in Gen V. What’s still the same with Muk? Well, its stats, for a start, are excellently distributed for a defensive Pokemon, with 105/75/100 being quite potent, as well as a very nice base 105 Attack stat to abuse. Poison makes quite a good defensive type, resisting arguably the most common attacking type in the game in Fighting, along with Poison, Bug and Grass types. Quite a lot for a single type (ok Steel, whatever). On the offensive side, things are pretty grim. A Gamefreak exec must have drank Bleach as a child, because they have had it in for Poison types since Gen I. Only one type is weakened SE by it, and it is resisted by a pretty hefty 4 types (Poison, Ghost, Rock, Ground), and Steel types are even immune! This means poison types have to rely on auxiliary attacking types to do any damage, and all but loose that essential STAB bonus. But fear not, because Muk has these in spades, having a good physical movepool, and a few status options that allow him to own certain sets. Whats new? Depressingly, very little. Its primary ability in Stench got a minute boost with an added 10% flinch rate when hit by a physical move. It’s no Serene Grace, but still. It also gained a new ability in Underhanded, but seriously, just use Sticky Hold or Stench. Looking over its moves, its seems Muk gained most of his most useful moves last gen, with the elemental punches, Focus Punch and Shadow Sneak helping round of his type coverage.
So, how about some sets?
Curse this Muk!
Muk @Black Sludge
Impish | Sticky Hold
252 HP/ 252 SpDef/ 4 Def
-Curse
-Shadow Sneak/ Shadow Punch
-Brick Break
-Ice Punch/Rest/Mean Look/ Whatever
Arguably Muks best set remains Muks bet set. Curse up to boost his already high Atk, and to build up his Def to allow him to take hits from both sides of the spectrum. Ghost and Fighting make up a combo that, even in Gen V, is unresisted. Shadow Sneak allows Muk to bypass the Spe drops caused by Curse and to hit Psychics that will be itching to get in at the sight of the blob. Shadow Punch, however, allows a higher BP alternative, if using a unSTABed 40BP move as one of your only attacking types makes you feel ill. In most cases, just go with priority. Ice Punch allows you to hit Grounds super effectively, since most have high Def, its better to try and remove them quickly before they can do some serious damage. However, also mentioned is Mean look, which I thought would be useful to allow Muk to come in on something it resists, and keep it in, allowing Muk a few turns set up.
The EV's focus on Muks defensive capabilities, since its going to be boosting its Atk and Def anyway, its better to augment his naturally Special bulk to allow him to do so.
Muk @Black Sludge
Impish | Sticky Hold
252 HP/ 252 SpDef/ 4 Def
-Curse
-Shadow Sneak/ Shadow Punch
-Brick Break
-Ice Punch/Rest/Mean Look/ Whatever
Arguably Muks best set remains Muks bet set. Curse up to boost his already high Atk, and to build up his Def to allow him to take hits from both sides of the spectrum. Ghost and Fighting make up a combo that, even in Gen V, is unresisted. Shadow Sneak allows Muk to bypass the Spe drops caused by Curse and to hit Psychics that will be itching to get in at the sight of the blob. Shadow Punch, however, allows a higher BP alternative, if using a unSTABed 40BP move as one of your only attacking types makes you feel ill. In most cases, just go with priority. Ice Punch allows you to hit Grounds super effectively, since most have high Def, its better to try and remove them quickly before they can do some serious damage. However, also mentioned is Mean look, which I thought would be useful to allow Muk to come in on something it resists, and keep it in, allowing Muk a few turns set up.
The EV's focus on Muks defensive capabilities, since its going to be boosting its Atk and Def anyway, its better to augment his naturally Special bulk to allow him to do so.
Muk this...
Muk @Black Sludge
Adamant | Sticky Hold
212 HP/ 252 Atk/ 44 Def
-Stockpile
-Substitute
-Focus Punch
-Shadow Punch/ Shadow Sneak
Ok, I did try hard to think outside the box with this instead of just a lazy Choice Band set. Think of it as SubPunch with a twist. Already blessed with the ability to make 101HP subs, Stockpile again allows Muk to boost both its defenses, which will hopefully make breaking his subs a very hard task indeed after a few boosts. Again, the Fighting/ Ghost combo is used here, since its STAB sucks and it doesn't need it anyway.
The EVs focus on maxing Atk, since Muk won't be boosting it.
Muk @Black Sludge
Adamant | Sticky Hold
212 HP/ 252 Atk/ 44 Def
-Stockpile
-Substitute
-Focus Punch
-Shadow Punch/ Shadow Sneak
Ok, I did try hard to think outside the box with this instead of just a lazy Choice Band set. Think of it as SubPunch with a twist. Already blessed with the ability to make 101HP subs, Stockpile again allows Muk to boost both its defenses, which will hopefully make breaking his subs a very hard task indeed after a few boosts. Again, the Fighting/ Ghost combo is used here, since its STAB sucks and it doesn't need it anyway.
The EVs focus on maxing Atk, since Muk won't be boosting it.
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So there we have it. Let me know if anything is amiss, or if I missed anything. Also, I suck at EVs, so if anyone wants to suggest better spreads, go right ahead.