Weezing and Muk
The last metas of ZU has seen one poison type -- Weezing come out on top, while Muk was relegated to niche strategies with Acid Armor. It's easy to see why: 85 Special hits ranges while 65 Special does not. Weezing crucially hits 3HKOs on Sandslash and Primeape, and gives pause to physical attackers like Onix and Rhyhorn from switching in. Furthermore, while not getting a clean 2HKO on Pidgeot with Thunderbolt, having better ranges to KO when accounting for Double-Edge chip is important. Muk didn't have much over Weezing barring Acid Armor, it is slightly bulkier on the special side and takes Seismic Toss better, but those calcs don't really push the needle. Mega Drain is cute but it is very easy to switch into, and access to Body Slam really doesn't mean much when there are better mons that click Body Slam in the tier. However, the recent drops seem to have shifted things around so that running non-Acid Armor Muk may actually have some merits.
Introducing: The Sludger
There is a big difference between 90 base Attack and 105 base Attack, and that's that Sludge gets 3HKO ranges with 105 base Attack. It has a
40% chance to poison, and is 100% accurate, unlike Fire Blast. Body Slam doesn't get these ranges so I don't think it's a replacement. The new drops have shifted the metagame a bit, so Muk is favored in some crucial matchups. The classic Weezing moveset is
Fire Blast
Thunderbolt
Explosion
Hyper Beam/Sludge
Using Muk with this moveset is possible but pretty cringe and you should honestly be running Weezing if you opt to use this set. However, this Muk set has merits:
Sludge
Thunderbolt
Hyper Beam/Mega Drain/ Acid Armor
Explosion
I think Fire Blast should be dropped on Muk. It hits terrible KO ranges, while Sludge hits real ranges that can be compared to Weezing. Even if you do have Fire Blast, many times you'd rather just click Sludge anyways on Muk. You can have 2 decent moves but you can only click 1 each turn, and spamming Fire Blast makes you wish you were actually using Weezing. Here are how Weezing and Muk differ in key matchups:
Dragonair: Muk is better

Weezing Sludge: (26.4 - 31.3%) -- guaranteed 4HKO
Muk Sludge: (29.2 - 34.4%) -- 4.2% chance to 3HKO. With a poison on the 1st or 2nd, it is a guaranteed 3HKO.
Dragonair eats both Fire Blast and Thunderbolt from Weezing, taking 17.8 max from Fire Blast. Furthermore, Weezing has a 53% OHKO with Explosion, while Muk's is guaranteed.
Magmar: Muk is better

Weezing Sludge: 92-109 (27.6 - 32.7%) -- guaranteed 4HKO
Muk Sludge: (30.6 - 36.3%) -- 57.8% chance to 3HKO. With a poison, it is guaranteed
Weezing does max 24.3% to Magmar with Thunderbolt. It's boom has only an 83% chance to OHKO. This pattern is roughly the same with other Fires, like Flareon, Charmeleon, and Ponyta, but the OHKO for Explosion on Flareon is 59%, while Muk is guaranteed.
Drowzee: Slightly Muk favored

I don't think this mon is particularly good in this meta, Muk has better rolls versus Drowzee, since Fire Blast does 30.6% max with Weezing. Weezing's Sludge almost always 3HKOs anyways and can afford to run it, but Muk can drop Fire Blast and run Hyper Beam, which snipes Drowzee from 54.7-64.3. Muk is slightly better, though I don't think it means much.
Parasect and
Butterfree: Slightly Muk favored

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Weezing has to hit the 85% accurate move to OHKO parasect. Weezing's Sludge has a 70% KO chance, with crits that's around a 77% chance. Muk has a 100% OHKO with Sludge, though you are probably hitting that range with Weezing anyways, and Parasect staying in versus a potential Fire Blast basically never happens, so they are about as good, though Muk has a slight advantage. Both Weezing and Muk cleanly 2HKO Butterfree with Sludge, but if Weezing opts for Hyper Beam over Sludge, Fire Blast accuracy and Special drops from Butterfree make it a lot more dicey. The solution is to run Sludge on Weezing since Hyper Beam doesn't hit too much, so this interaction is about equal for both of them.
Scyther: Slightly Weezing Favored

Muk's Thunderbolt doesn't 2HKO Scyther, but Sludge does. Weezing's Sludge doesn't 2HKO Scyther, but Thunderbolt does. It's basically a wash, but Weezing takes max 36% from Hyper Beam while Muk takes 39% max, and also Thunderbolt can paralyze 10% of the time.
Omanyte: Hard to tell?

The two neutral buttons that Weezing and Muk click, Fire Blast and Sludge, bounce right off of Omanyte. However, Weezing has a 2HKO with Thunderbolt, while Muk does not. Weezing's Thunderbolt does from 49-57.3, while Muk's hits from 41.7-49. However, Muk's barely perceptible Special bulk comes in handy. Omanyte's Surf is a 95% chance to 3HKO Weezing, while it is only a 31.2% chance versus Muk. Finally, Muk has one final trick up it's sleeve: Mega Drain.
Muk Mega Drain vs. Omanyte: 98-116 (35.8 - 42.4%) (11.8 - 14% recovered) -- guaranteed 3HKO
Thunderbolt 3HKOs Omanyte anyways, and the extra healing makes Omanyte's Surf do a measly 20%. However, it is important to note that both Poisons outspeed Omanyte, so perhaps the cleaner range that Weezing offers is more important. But maybe you really need to 1v1 Omanyte, so Muk would do better, or perhaps your Poison is paralyzed, so the 3HKO ranges become really important. Either way, I don't feel comfortable making a call here.
Primeape: About the same

Muk Sludge vs. Primeape: 100-118 (30 - 35.4%) -- 25.2% chance to 3HKO
Weezing Fire Blast vs. Primeape: 108-127 (32.4 - 38.1%) -- 97.8% chance to 3HKO
With a poison, Muk's Sludge rate increasing drastically based on when Primeape gets Poisoned. Fire Blast is 85% accurate, and the burn chance isn't particularly relevant because it Primeape is probably clicking Seismic Toss into Weezing, which does 30%. Muk takes basically nothing, capping out at 24.2% of the time. Lastly, Weezing's Explosion isa 59% roll to KO, while Muk's is guaranteed. On one hand, Weezing actually wins more often than Muk, which really wants a poison in one of those three sludges (happens 78% of the time). However, Weezing can absolutely drop the ball and miss crucial Fire Blasts. While burns aren't relevant in this calculation, getting a burn could cripple Primeape's ability to stuff later in the game. It's a tough call.
Tentacool: Slightly Weezing Favored

Weezing Thunderbolt: 125-148 (44.1 - 52.2%) -- 16.8% chance to 2HKO
Muk Thunderbolt: 107-126 (37.8 - 44.5%) -- guaranteed 3HKO
Weezing has a paltry 17% chance to KO, but Muk doesn't really have it better. Sludge -> Hyper Beam doesn't KO, so you'd need to Sludge twice into Hyper Beam, which is way easier to play around. At that point you just click Thunderbolt like Weezing. However, Muk can opt to run Body Slam, and Sludge -> Body Slam -> Body Slam KOs. You have to drop Hyper Beam for this, so it's a tough call that probably isn't worth it. Still, I'd say Weezing has the definitive edge here.
Weezing and
Muk: Weezing is better

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Weezing hits on the Special side, so it doesn't care about burns, and it outspeeds Muk. Weezing also resists Sludge and cannot be poisoned, and Muk's Explosion does 79% max. Obviously, Weezing is better in the mirror.
Sandslash: Weezing is a lot better

Weezing Fire Blast: 111-131 (31.4 - 37.1%) -- 82.9% chance to 3HKO
Weezing has a realistic chance to 3HKO Sandslash, and can burn it. Swithing Sandslash into Weezing is kind of a gamble once you consider the burn chance, but Sandslash eating a Sludge poison really doesn't accomplish much. It could turn into a sleep blocker. Weezing lives EQ at above 68, and Muk does so from 74. Muk's boom hits 80% while Weezing's hits 72%. They are both pretty bad into Sandslash to be honest, but crucially, Sandslash can't just switch into Weezing with impunity, so it's a clear Weezing W.
Pidgeot: Weezing is way better, but it's not that bad.

The biggest reason why Weezing >>>>> Muk in the past is because of this mon. The advantages with Muk are sometimes better, but in the old meta this mon was on every team, and the better Thunderbolt range that Weezing offered, alongside better physical bulk made Weezing the superior choice. Muk obviously can't compete, but I'd argue that this interaction is why less important in this version of ZU. Magmar is now a really good mon in the tier, which outspeeds Pidgeot. Omanyte is now a really good mon too, and it walls Pidgeot quite well, and unlike Onix and Rhyhorn, Omanyte's Surf and Blizzard hit like a truck, so this is a big deal. Lastly, Scyther is now here. If you are going to run a mon walled by Omanyte, you'd want a mon that actually outspeeds everything, too. Pidgeot simply doesn't have as privileged of a position in the tier anymore, it is completely droppable, and arguably not even top 10.
Conclusion
Muk is actually superior in many cases to Weezing, having better KO ranges versus Dragonair and Fire-types, while its Explosion always hits the OHKO in certain matchups. Furthermore, it has better KO ranges with Hyper Beam, and can live 3 Omanyte Surfs. While it doesn't have Fire Blast burns to deter Sandslash, it makes up for it in being 100% accurate and a 40% poison rate. Lastly, Muk has actual set variety. They could be running Mega Drain or Hyper Beam or even Acid Armor, with the last one greatly changing how you deal with Muk. These two mons are separate from each other: Weezing doesn't have to fear Sandslash switching in for free, while Muk has a better matchup spread and is generally more consistent in both Accuracy and Explosion ranges. I honestly don't know which is "better," but I believe that Muk is a genuine choice, even without Acid Armor.