Toxic.
Seriously, this move is great against anything with a ton of HP or defense, and useful against anything that you don't have a type advantage against. It's also useful if your own pokemon becomes confused/sleep/frozen/etc. as it continues to deal increasing amounts of damage.
Combo'd together with leech seed, it chews through heavy hitters like Dragonite or Gyarados.
I haven't played since Generation III though, so it may not be as effective nowadays.
Seriously, this move is great against anything with a ton of HP or defense, and useful against anything that you don't have a type advantage against. It's also useful if your own pokemon becomes confused/sleep/frozen/etc. as it continues to deal increasing amounts of damage.
Combo'd together with leech seed, it chews through heavy hitters like Dragonite or Gyarados.
I haven't played since Generation III though, so it may not be as effective nowadays.