Here's a list of the most poisonous chemicals known.
Toxicity varies from one species to another (i.e., what may be poisonous for a mouse may be more/less poisonous to a human) and within a species (i.e., age, sex, genetics all affect susceptibility to a toxin).
#2 botulinal neurotoxin (bacteria)
#3 shigella (bacteria)
#3 palytoxin (coral)
#4 diphtheria (bacteria)
#5 ricin (from castor beans)
#6 aflatoxins (mold which grows on nuts, legumes, seeds)
1-784 micrograms, depending on type of aflatoxin