Poison types are mainly weak to Ground and Psychic attacks in modern Pokémon games. In most formats (like the main series and Pokémon GO), those are the only two standard type weaknesses for pure Poison Pokémon.

Core weaknesses

  • Ground-type moves hit Poison for super-effective damage (2×), making powerful Ground attackers some of the best counters.
  • Psychic-type moves also deal super-effective damage to Poison, so strong Psychic attackers are reliable answers as well.

Quick gameplay notes

  • Many Ground Pokémon also resist Poison, letting them switch into Poison moves more safely.
  • Psychic types do not usually resist Poison, so while they hit hard, they can still take decent damage if they are slower or not very bulky.

TL;DR: Poison types are weak to Ground and Psychic moves in current Pokémon type charts.