The Brazilian Wandering Spider (Phoneutria nigriventer) is regarded by some as the most dangerous spider in the world. It is highly venomous and not reluctant to attack people who appear threatening to it. However, recent studies indicate that it only injects venom in about one third of its bites and may only inject a small amount in another third. Thus the effects of the bites from this spider can range from only a couple of pin pricks to a full blown envenomation. The Sydney funnel-web spider apparently injects venom in every bite and may thus in effect be more dangerous.
External link for a beautiful picture of the underside of this visually attractive spider:
