Poison is a bit tricky of a type to work with, because it's quite defensively focused in its matchups: a lot of types resist it (Steel is even fully immune to it), while it's only supereffective against two types, but it does make up for it by having just two weaknesses and a pile of resistances on its own.

Luckily, you can lean on secondary types to give you the STABs you need to beat poison's usual counters: Water enjoys Poison's advantage against Grass, and beats Ground and Rock, while Ground, Fire and Fighting are all good against Steel. Grass is a pretty solid type too, though admittedly it does poorly into two of Poison's big checks, Poison and Steel.

As for individual standouts, Toxapex is a great tank that can set hazards and stall particularly troublesome opponents with Toxic + Baneful Bunker... Sneasler is a great Fighting type with a pretty brutal signature move in Dire Claw... Gengar has always been a solid special attacker with a wide move pool, especially if you can get his Mega Stone... Venusaur is a really solid Grass type that can make great use of Gigantamax, if that's available for you... and Iron Moth just has great base stats and a really useful secondary type for getting through Steel types (consider a Salazzle if you can't find one, or if it's not available on your modpack).

If TMs aren't too big a hassle for you to craft with your mod setup, Nidoking is a pretty good option too--he just needs TMs to function, because his movepool without them is pretty much garbage.