What Is The Most Dangerous Substance In The World?

If we are talking the most dangerous man made poison, then I would say dioxin. In relation to cyanide, dioxin is 60 thousand times more toxic than it. It only takes a dose of 50 micrograms to kill a human being.

You can find an article on it here.

Botulinum toxin,by far the most lethal toxin there is, a few kilo's is enough to kill all people!

Dimethyl mercury

Actually it is something called Biological Agent VX it is deemed a weapon of mass destruction

Diethyl Azodicarboxylate A.K.A DEAD...it is explosive; shock sensitive; carcinogenic and eye, skin and respiratory irritant

The answers above are completely ignorant of chemistry, toxicology, Wikipedia, and Google.
There are many classes of toxins depending on their mode of action.
Generally, batrachotoxin is considered one of the most toxic materials, if not the most toxic.
Dioxin is not very toxic and is used on diagnostic testing; it was one of the first materials to prove that it is NOT toxic until the cell's receptor sites are filled.
One theory, the 'one-hit' theory, holds that a single virus or hit by single ionizing radiation photon or particle could produce a fatal cancer.

I would say any type of acid.

Depends on what you define "dangerous" and "substance" as... If by substance you mean physical matter, then emotions (natural ones, not chemical-induced), micro black holes, etc. Don't really count. Assuming "dangerous" means it requires a small amount and kills quickly, then I would have to say it is polonium.

It has an LD-50 of 10 to 50 billionths of a gram per kilogram of body weight, so a single gram of vaporized polonium should be more than enough to kill off several hundred thousand people.

In the making of computer chips the Warning planks read 4 4 4!
This shows the highest level OSHA recognizes, and the only place I've ever seen them all maxed out! Their are several able to be in your group, BUT one really stands out, It's Olem!
When you start off with Sulphuric Acid, and concentrate it to 100%, it changes into "Olem", and has all the same effects of the original with the SO2 and SO3 gases being a lot worse!
HOWEVER NOW it is shin absorbed, and WILL go to the bone morrow on contact, and this effect CAN NOT be washed off and an a shot is needed very fast to stop it, and kept in the area by OSHA's rules, because! No shot and your DEAD! Now that's Dangerous!
Logic101

I guess it is all relative to who you are, what you are, how old you are, where you are, and all those things related to you. Rather I think that it is not poison or danger that kills or harms us. It is we who let them express and influence us. Virtually even we are a poison to that compound's existance...          So, danger or fear or poison are all host specific, not the compound...                               
                                                                -PRATIK S KALAWALA
                                                                          B.Sc CHEMISTRY(major)
                                                                              M.S.UNIVERSITY (BARODA)
                                                                                  GUJARAT.
                                                                                        INDIA.(jai hind)

Actually, the most dangerous substance in the world could be regarded as the micro black hole they have been flirting with in the European accelerator project. If accidentally produced it has the potential to absorb our solar system! Now THAT is dangerous.