Generally the answer given will be Plutonium. However you use the term ''most dangerous''. Its a looser definition as to what is the most dangerous.

Plutonium is both toxic chemically and because of its ionising radiation and has often been called "the most toxic element known to man". However, if we take ''toxic'' to mean - a poison with a fairly fast, often fatal, effect - then scientists do not consider plutonium very toxic at all.

For radioactive materials, the words “toxic” and “toxicity” describe the ability to damage biological material - not how fast it kills people. Plutonium emits alpha, beta and gamma rays and low-level exposures may not be detected easily or differentiated from other kinds of radiation poisoning.

There are substances in daily use that, per unit of mass, have greater chemical toxicity such as arsenic, cyanide, caffeine, or botulism.

Arsenic is the 20th most common element in the earth's crust and readily gathers in ground water making it fairly common. 1 mg/kg/day will kill an adult.

Cyanide is less common in the environment but almost as deadly. It is in industrial fire smoke and Cassava roots - which must be washed before consumption. 1.5 mg/kg body weight will kill an adult

Caffeine is a very potent poison and is widely available. Caffeine overdoses are relatively common in the United States. 10g will kill an adult. The US FDA has limited the maximum dose in a food to be 200mg. 50 of those will kill an adult.

Botulism - Clostridium botulinum toxin – is produced by a bacteria and is the most deadly by volume at 0.005–0.05 µg/kg to kill an adult. Its is used in cosmetic procedures to kill nerves in the face and thereby remove wrinkles by relaxing the muscles. It grows in poorly sealed bottled fruit or cans and depends on the absence of oxygen to grow.

And lets not forget that old forgotten dynamite someone left lying around - which is very dangerous due to the glycerines that leak from the dynamite and make it highly unstable when handled.

Plutonium is the most dangerous element in the World along with hydrogen and uranium as they are used in the making of a nuclear bomb. In a different way then maybe acid in their natural state-sulfuric, etc.. Also poisons boiled from roots and evergreen needles. This could also include ricin which comes from castor beans. In terms of animals batrachotoxin from a frog in South America but also gila monsters and kimodo dragons that kill by bacteria poisoning from dirty mouths. So I guess you could also include the snakes from Australia and the box jelly fish. There are so many ways to die in this World of ours.