What is the most deadly toxin known to science? Both natural and synthetic?
Ricin, from castor beans may qualify as the most potent toxin. Use “ricin” in any search engine for more info.
A toxin is a poisonous substance produced by an organism. By definiton, it’s natural. If you’re referring to any substance man-made or otherwise, I think it breaks down into the following categories. (Bear in mind, that the relative lethality of various substances is subjective.)
Toxin (including venoms): Ricin
Chemical Agent: VX, a nerve agent (.01 gm)
Biological Agent: Anthrax
Element: Plutonium (micrograms)
Immunization against Ricin and Anthrax is possible. Wear a gas mask and VX will dissipate. Plutonium, however, has a half-life of about 24k years. You can make a case for any of these, and others.
Okay where does botulin fit in here? I know I’ve heard it touted as the most lethal neurotoxin many times.
I think that saxitoxin as purified from the dinoflagellates which cause red tides holds the record for lowest LD50. It’s a very potent sodium channel blocker. Cholera toxin is also pretty good, with a few molecules serving to kill the average cell.
If you want to talk strictly in terms of LD50 - the quantity of toxin necessary to cause death in 50% of infected subjects, it’s Botulism, no question. Here’s a list from “Medical Management of Biological Casualties Handbook” from USAMRIID. Accueil - L'univers du chien However, this list ignores such improtant details as the resilience of the microbe, manner of delivery, and likelihood of secondary infection. Accounting for all variables, Anthrax tops THAT list, at least according to the OSD. http://www.anthrax.osd.mil (I said this was subjective) 
List below shows agent, LD50, # of microbes, source.
The toxins produced by Bacillus anthracis are usually produced after the microorganism has entered the victim and started multiplying. There are two common and one rare forms of anthrax, being cutaneous, inhalation and intestinal. Correspondingly, the most common means of transmission are person-to-person or animal-vector-to-person, via a cut or abrasion, inhalation or ingestion. The disorder which Bacillus anthracis causes is an infection. Bacillus anthracis may well be the most potent infective agent, but my money would be on smallpox. Or one of the new GM nasties.
I am aware of seven types of Clostridium botulinum, A to G, which vary according to their source, (fish, fowl, meat, veggies etc). Their major toxins vary in their effects on humans, with a couple not really affecting humans at all. [British accent] Demn decent of them, what?[/British accent]. Used to be that food-bourne botulism was the major hazard, with wound botulism lagging well behind, and infant botulism a relatively recent discovery. However, changes in food processing hygiene and drug use have changed that.(Lets leave wound and infant botulism out of this discussion - they are whole other threads, and I’ll loose any hope of controlling my parentheses!).
In the good old days, when botulism was a foodbourne illness, the botulinum toxins were almost always produced outside the host under anaerobic (or at the least, locally reducing) conditions, and had their effect when ingested. ‘It’ is not spread by person-to-person or animal-vector-to-person transmission (unless you count someone handing around dud food at a party!). Botulism is an intoxication.
As far as I know, one of them thare botulinum toxins is the most powerful toxin known.
Canape anyone?