Why is Botulinum toxin the most potent poison known?
The biochemical mechanism of BT is pretty well laid out at Wikipedia already. The toxin is likely very potent because (1) it is targeted to neurons; (2) neurons endocytose BT; and (3) the BT light chain's protease activity is able to cleave a wide range of SNARE-complex proteins which effectively disrupt the neurons ability to secrete monoamine neurotransmitters, and peptide hormones. Basically, the neurons are completely inhibited from transmitting nerve impulses.