There's a lot of venomous snakes in the world but the Inland taipan (Oxyuranus microlepidotus) is considered the most venomous snake in the world with a murine LD 50 value of 0.025 mg/kg SC.
It's really not the snake itself though that's poisonous but instead it's the venom from the snake that is poisonous to humans and other animals should they get bitten by the snake.