The most deadly snake is the one you do not see.
Ld50 aside as its not the best source as I do not know of enough humans to willingly be Bitten and die to find out the true effects of venoms.
North American Snake Envenomations
Dedra R. Tolson, M.D.
Clinical Instructor/Attending Physician,
Emergency Medical Service
University of Washington Medical Center-
Madigan Army Hospital Emergency Medicine Residency Program
Incidence: Approximately 20 of the 120 snake species in the United States are venomous. Snakes inflict approximately 8,000 bites each year in the United States of which 2,000 are inflicted by venomous snakes (1). Pit vipers are responsible for the majority of North American snakebites. Since reporting of snakebites is not mandatory and many victims do not seek medical attention, it is assumed that many snakebites are not reported. Thus, the incidence of bites is probably much higher. The majority of bites occur in the southern United States during the summer months when snakes and victims are most active. Most snakebites occur on the extremities and alcohol intoxication is a common factor leading to envenomations (2). With the availability of antivenom and advances in emergent medical care, mortality rates today are below 0.5 percent (3). Approximately 5-10 deaths per year occur as a result of poisonous snake envenomation (4). Most of these occur in children and the elderly.
http://www.emedhome.com/features_arc...ail.cfm?FID=99
Who snakes bite: It has been estimated that 5 million snakebites occur
worldwide each year, causing about 125,000 deaths. Snakebites are more common in tropical regions and in areas that are primarily agricultural. In these areas, large numbers of people coexist with numerous snakes. Five to ten deaths occur per year from snakebite in the United States.
People provoke bites by handling or even attacking snakes in a significant number of cases in the United States. Of the estimated 45,000 snakebites per year in the United States, about 8000 are by venomous snakes.
http://www.emedicinehealth.com/articles/10707-1.asp
Based on world medicine the more deadly bites would be from third world countries with improper medical care systems. You would be far less likely to die in the United States or Australia then you would in say Africa. As to which is most venomous I would go with the King Cobra followed by Gaboon viper, Rhino viper, Puff adder and Eastern Diamond Back for potential venom yeilds maybe not in that oreder but deffinately my top 5.