For Poison, there just isn't a really a Pokemon with a good stat spread who is Poison. No Poison type Pokemon has a base attack of over 105 which is underwhelming entirely.
The best Poison types, Tentacruel, Roserade, Gengar have special niches not relate to stat spread.
Tentacruel has Rapid Spin and is part Water which always helps.
Roserade has Toxic Spikes and Sleep Powder plus a fairly good spread but not amazing.
Gengar is part Ghost, has Levitate and has a great stat spread but not to take advantage of what Poison brings, he'd be just as good without Poison typing.
Even Crobat, the highest BST, has a problems brought on by being Flying and underwhelming offensively and defensively despite being very robust and fast. He's arguably the best representation of a Poison type.
If there was a BST 600 Poison type, with solid secondary typing AND a good movepool, no one would complain at all.
The best Poison types, Tentacruel, Roserade, Gengar have special niches not relate to stat spread.
Tentacruel has Rapid Spin and is part Water which always helps.
Roserade has Toxic Spikes and Sleep Powder plus a fairly good spread but not amazing.
Gengar is part Ghost, has Levitate and has a great stat spread but not to take advantage of what Poison brings, he'd be just as good without Poison typing.
Even Crobat, the highest BST, has a problems brought on by being Flying and underwhelming offensively and defensively despite being very robust and fast. He's arguably the best representation of a Poison type.
If there was a BST 600 Poison type, with solid secondary typing AND a good movepool, no one would complain at all.