The most toxic by weight is almost universally acknowledged to be botulinum toxin, from Clostridium Botulinum (of botulism fame), the LD:50 is just insane, one figure quotes as 1ng/kg, and acts by blocking the release of acetylcholine at the nicotinic receptors (neuromuscular junction subtype)
which has the same end effects of neuromuscular blocking agents such as succinylcholine, suxamethonium, pancuronium and curare.
Does it long term too, which makes it useful in really, REALLY small dosages for cosmetic procedures.
Nerve agents (organofluorophosphate esters, including garden pesticides, G-agents, V-agents and novichok agents in increasing order of toxicity)
act by blocking the action of cholinesterase enzymes, acetylcholinesterase, and in the case of novichoks, acetyl- and butyrylcholinesterases, the G-agent soman, unlike most G, and V agents, also "age" the enzyme, within minutes, compared to days with VX, rendering oxime treatment to reactivate poisoned cholinesterase enzymes.
Novichok agents are worse, they cannot be treated with oximes.
Ricin is an interesting one, as its a protein (like botulinum toxin), known as a toxalbumin, and related to the more toxic abrin, and other toxalbumin type proteins present in mistletoe species, these nasties act by inactivating protein synthesis, via the ribosomes present in cells.
Vey slow, very lethal, and very unpleasant way to go too.