A point of clarification --- volatile does not refer to explosive or combustive. It refers to the ease with which a liquid evaporates. Gasoline is volatile. Water is not as volatile. Your question is tough to answer, but my own personal choice of two things I would not mix are pure oxygen and aluminum powder, because when aluminum starts to burn (it is sometimes called thermite), the heat is so intense, and the temperature so high that the fire is almost impossible to put out. But there are many chemical compounds (explosives such as RDX) which are dangerous in themselves.

Heh - RNA and phospholipids. Given enough time, it creates life, which can do all sorts of things!To be honest, it depends on what you mean by 'dangerous'. Do you mean explosive? I think that the most powerful chemical explosive known is cubane (eight carbon atoms bound to each-other in a cube), with a nitrate group on each carbon.

The reason why that is so explosive is because it is extremely unstable, and would be in a much lower chemical energy state in a different form, and could release a great deal of thermal energy in the transition. Or do you mean toxic? The deadliest toxins are toxins that interfere with our enzymes somehow, interacting with key molecules. I don't know which toxin is the worst, but there are an awful lot that are very bad!