What's the most poisonous snake?
It's hard to be sure; the only way of evaluating that would be to have lots of different snakes bite lots of different people and see which species had the highest kill rate. For obvious reasons, this has never been tried. With that said, the strongest contender is the fierce snake, or inland taipan, of Australia. Fortunately, there is an antivenom available for it.
(Strictly speaking, snakes are venomous,> not poisonous. Poisons generally work though ingestion; venoms, through injection. In other words, if you bite into it and you get sick, it's poisonous. If it bites into you and you get sick, it's venomous. That said, both terms can be—and often are—used more loosely.)