Cyanide is the granddaddy of synthetic poisons.

A dose as small as 50 milligrams (ingested) can kill an adult man.

Easily made from common chemicals, cyanide is used as is for poisoning bullets and food, and is used in the making of Hydrogen Cyanide, Cyanogen Chloride, Tabun, and numerous other poison gases.

Ricin is just about the easiest, and at the same time, most toxic poison that a criminal can make. Less than a milligram (1/1,000 of a gram) injected or inhaled will kill a person several times over. For individual killings, it has the advantage of being undetectable in toxicology scans since the poison is a catalyst the starts a chain reaction in the body, and is destroyed  before the symptoms begin to show.

With properly sized and dispersed dry particles, ricin is at least 10x more toxic than the most potent nerve gas. A 1% water solution atomized with a small explosive burster has the same effectiveness as sarin nerve gas. The only disadvantage ricin has is the time it takes for the victims to die is about 1 – 2 weeks. So you won't have the quick tactical effect of nerve gas. But this can also be good in that, using a covert dissemination, the criminal has time to escape before the attack is detected.

This book comprehensively details how to produce both of these deadly poisons out of commonly available materials and ingredients.