For early game Trubbish was great. I was very surprised by its speed. Mine outsped most enemies, plus Toxic Spikes - Sludge - Acid Spray at early levels helped me a lot with Florinia and even up to Shelly it was quite useful. After Shelly however it was boxed and not looked at again.
Later ingame, around the 6th gym you can get a Good Rod to fish up a Skrelp, Dragalge helped me a lot from Noel and further. Personally very little experience with other poison types, I love Roserade but since there's no TM for Sludge Bomb in game it needs breeding for the good moves.
I think Muk is a reliable Poison type too with learning Sludge Bomb and Mud Bomb by level up, and Crobat of course can be a beast. The Nido's are also only available after the 5th Gym.
Venomoth, is great for support but needs a bit of a set up with Quiver Dance and relies heavily on TMs, Scolipede is a fast, reliable physical attacker with Steamroller/Megahorn and Poison Tail(Poison Jab TM isn't ingame) and perhaps Rock Climb, and Ariados doesn't have the greatest stats but it has some great moves with Poison Jab, Shadow Sneak, Sucker Punch and Sticky Web by levelup.
Swalot is a lot of fun to work with and I hear great stuff about Toxicroak. Vileplume, Dustox and Amoonguss are great to work with from slight experience in the main games, but again, TM's bruh.