Are platypuses the most poisonous animals in the world?

No, they are not the most poisonous animals in the world, but they are one of the world's very few venomous mammals.

No one has been recorded as dying from platypus venom, but people who have been "spurred" by a platypus report that the pain is strong enough to cause vomiting that may last for days, weeks or sometimes even months. The pain cannot be relieved by morphine and other standard pain-killing drugs. It seems the only way it can be relieved is through anaesthesia of the main nerve from the spur site.