KalosianPorygon asks:

What is your ranking of Poison-type Pokémon from less to most deadly?

I’m assuming you mean specifically in terms of how poisonous/venomous they are, what kind of LD₅₀ we might be looking at for the various sorts of awful $#!t they throw around; that sort of thing.  Well, there are at present 69 (…nice) Poison Pokémon, so I hope you’ll not mind if I just go for a quick top 5… We can probably eliminate unevolved Pokémon right off the bat; that narrows it down to 32 (give or take).  What, then, can we use as measures of lethality?

• We can look at the animals (or substances) that inspired the Pokémon and how deadly those are.  Vipers and jellyfish?  Some species can very well drop a real human with alarming speed.  Bats?  Vampire bats are arguably sort of “venomous” because of their anti-coagulant saliva, but a normal dose of that isn’t going to kill anything.
• We can look at any poisonous moves or abilities the Pokémon have.  Just about any Pokémon can be taught Toxic, and almost all Poison-types can be taught Sludge Bomb, but not everyone learns these moves “naturally,” and many Poison-types also have poison-related abilities like Liquid Ooze and Poison Touch.  Stats, I think, might be misleading; a Pokémon like Toxapex with lowish attack stats might have slower-acting but nonetheless potent venom.
• We can look at what the Pokédex tells us.  Some Poison Pokémon’s flavour text barely mentions poison or venom, or imply that the Pokémon relies more on its physical strength; others give us explicit descriptions of what the Pokémon’s poisons are capable of.

On those grounds, I present what I have judged, in my infinite wisdom, to be probably the most toxic of all Poison Pokémon:

#5: Beedrill

Now, don’t get me wrong, Beedrill is a rubbish Pokémon (barring mega evolution, of course), and the Pokédex is not excessive in its descriptions of Beedrill’s venom.  However, it has a number of nasty Poison attacks and is the only Poison Pokémon with access to Adaptability, which gives its Poison moves a significant boost in power.  It’s also based on the Japanese giant hornet, an absolute bastard of an insect with one of the most painful stings in the world (although it takes several stings from multiple hornets to actually kill a non-allergic person).

#4: Roserade

Although rose thorns aren’t actually poisonous, and Roserade only has a few significant Poison-type moves (most significantly Toxic Spikes) plus the Poison Point ability, it gets a spot here because of the Pokédex’s fairly detailed description of its capabilities.  Roserade has two different poisons that come from its two flower bouquets.  One of them is fast-acting and the other is slow-acting – but both are described as “near-fatal” or (in Shield) “life-threatening,” which is a pretty bold statement for the Pokédex.

#3: Dragalge

Leafy sea dragons aren’t poisonous, but the Pokédex claims that Dragalge’s poison can “eat through the hull of a tanker.”  Now, frankly that sounds like it’s describing acid and corrosion, not poison, which is why Dragalge isn’t in first or second place, but even if we ignore the Pokédex’s bull$#!t, Dragalge has not one but two poison-related abilities (Poison Point and Poison Touch), plus several nasty poisoning moves.

#2: Muk

Everything about Muk is poisonous; its footprints are toxic, its touch is toxic, it spreads disease and corruption wherever it goes (notwithstanding Alolan Muk’s charitable efforts to clean up the islands), and it has a long list of toxic attacks.  Muk’s not really based on any specific, identifiable kind of sludge or waste chemical, but industrial waste generally can have a dizzying range of awful effects, up to and including cancer.  Muk’s pretty awful, but it’s not #1, mainly because the Pokédex talks as if its poisonous properties are sickening rather than lethal – unlike the Pokémon in first place.

But first, a dishonourable mention: Eternatus

Despite being the one and only legendary Poison Pokémon, Eternatus has relatively few Poison attacks and doesn’t use poison any more effectively than other Pokémon.  It’s also… not even super-clear why it’s a Poison-type, exactly?  It could be that we’re supposed to associate Eternatus with corruption or pollution, but it doesn’t actually do anything to back that up.  Nothing about its design or its role in the story of Sword and Shield would be particularly incongruous if its type were pure Dragon, or Dragon/Dark, or even Dragon/Ghost.

And the last-place Pokémon for poisonous lethality: Victreebel

Pitcher plants aren’t even poisonous, let alone dangerous to anything larger than a hamster.  Victreebel is only a Poison-type because of the pitcher plant’s dissolving digestive acid, which… well… is something most vertebrate animals also have, including humans.  Unlike Roserade, though, Victreebel goes on to back this up with a very small selection of weak Poison attacks, no poison-related abilities, and no mention of poison in any of its flavour text (Weepinbell’s does mention the Poison Powder attack).  There’s a bit of semantic debate to be had on exactly where the lines are drawn between poisons and venoms and toxins and acids, but personally I think acid makes more sense under Water or even Grass (like Appletun’s Apple Acid) than under Poison.

But that brings us, finally, to…

#1: Toxicroak

Toxicroak’s Poison-type move list is not super long, although it does have the Poison Touch ability, which I rate quite highly for purposes of whatever this thing I’m doing is.  However, Toxicroak is based on one of the most poisonous animals in the world – the poison dart frog.  The deadliest species, Phyllobates terribilis (when an animal the size of a walnut gets the species name terribilis, you know it means business), has enough contact poison in its skin to kill ten adult humans, which kicks the $#!t out of any venomous snake or spider.  That doesn’t necessarily translate to Toxicroak – but in fact, the Pokédex does confirm that Toxicroak’s Poison Touch is so lethal “even a scratch could prove fatal.”  The Pokédex is generally pretty reluctant to talk about Pokémon powers actually killing anything, so when it does, I give it a lot of weight, and I’m willing to believe that Toxicroak is every bit as poisonous as its real cousin.