Almost all spiders are venomous, with very few exceptions. However, of more than 50,000 known species worldwide a handful are significantly venomous to humans.
The standard measure of toxicology is the LD50 test (basically a lethal dose which kills more than 50% of your test animals, usually mice or rats) the two species of spider are top of the league; one of the so called Brazilian wanderers, Phonutria fera, and the Sydney Funnel Web, Atrax robustus. That said death is the most extreme reaction to the venom of these spiders and the amount of venom injected varies. The recorded human deaths for these species over the last 50 years appears to about 15-20 each.
To put this in perspective - without doubt there are a greater number of deaths attributed each year to any colour car you care to name. Golf balls are probably also more dangerous.
That's interesting, Bombuslucorum
Of course it wouldn't be ethical to give humans a toxicology test; but mice and rats may not be sensitive in the same way to the same poisons as us[*]. Is there any means of allowing for that?
[* For example, I have heard that chocolate is poisonous to dogs; and a normal human dose of paracetamol may kill a cat as big as a lion]
The Brazilian Huntsman is not the most poisonous spider in the world, that would be the Brown Recluse.