To paraphrase the renowned venomous reptile expert Dr. Bryan G. Fry, "The world’s deadliest snake is the one that just bit you. Otherwise, you’re just arguing semantics."
There are several ways to classify the question, "What is the world's deadliest snake?" If one is asking what the most toxic snake in the world is, that would be the Inland Taipan (Oxyuranus microlepidotus). The Inland Taipan, also known as the Small Scaled Snake and Fierce Snake, is native to Australia, our lovely neighbor to the south who happens to be saddled with 80% of the world's venomous snake species. Although highly venomous, like most snakes it is shy and reclusive, biting only if threatened.
The Inland Taipan's venom consists of Taipoxin and protease enzymes, with each bite injecting an average of 44mg of venom. The venom of the Inland Taipan is 200 to 400 times as toxic as that of most rattlesnakes, and fifty times as toxic as that of most cobra species. The venom is a potent neurotoxin that could potentially kill an adult human in forty-five minutes. Owing to the snake's reclusive nature, all current positively reported bites have been herpetologists handling the snakes for study, and there have been no documented human fatalities since the development of Taipan Antivenin. Commonwealth Serum Laboratories in Australia currently produces an antivenin for both species of Taipan, the Common Taipan (Oxyuranus scutellatus) and the Inland Taipan (O. microlepidotus).
If one is asking what most herpetologists consider the most dangerous snake to be in close proximity with, that would be the Black mamba (Dendroaspis polylepis), due to its speed, agility, temperament, and venom toxicity. The Black mamba is native to Africa, and is one of the longest venomous snakes in the world, with average specimines reaching 2.5 meters, and some rare specimines growing up to 4.3 meters. Its name is derived from the black coloration of its mouth tissue, not from the color of its skin, which varies from dull yellow-green to gun-metal gray.
The Black mamba's venom consists mainly of neurotoxins. Its bite delivers an average of 100-120mg of venom, but in some occurrences it can deliver up to 400mg. The mortality rate from its bites is nearly 100%, unless the victim is promptly treated with antivenin. The Black mamba is also the fastest snake in the world, with some specimens clocked at 5.4 meters per second. These factors, along with the size of the Black mamba, make this species extremely dangerous to work with in captivity. However, bites in humans are an extremely uncommon occurrence. Currently there is a polyvalent antivenom produced by SAIMR (South African Institute for Medical Research) to treat Black mamba bites.
If one is asking about the snake that is responsible for the most fatalities per year, that dubious distinction goes to the Russell's viper (Daboia russelli). The Russell's viper, native to Southeast Asia, is named in honor of Dr. Patrick Russel. An average of about 1,000 snakebite fatalities per year are caused by the Russell's viper.
The average quantity of venom produced by individual specimens is erratic, yet considerable. Reported venom yields for adult specimens range from 21-268mg, with an average of 147mg. Envenomation symptoms begin with pain at the site of the bite, immediately followed by swelling. Bleeding is a common symptom, especially from the gums. There is a drop in blood pressure and the heart rate falls. Severe pain may last for 2-4 weeks, and death from septicemia, kidney, respiratory or cardiac failure may occur as long as 14 days post-envenomation. The Haffkine Institute in India currently produces a polyvalent antivenin for this species.
Dr. Bryan G. Fry maintains and excellent page on the LD50 of different snake venoms with dosages sorted by route of administration. Dr. Fry's venom page is located at http://www.venomdoc.com/LD50/LD50men.html