Photo from AFPMB
The Box jellyfish, called Sea Wasp (Chironex fleckeri) is the deadliest species of jellyfish. Also it is wide regarded as the deadliest creature ever. They can kill human only in 3 min and the sting is extremely painful.
Few notes of interest: First, they look like jellyfishes, but they are not actually real jellyfishes from the systematics' point of view. And second, there is antidote and few treatments which may help the victims survive the sting and I recommend these interested in the issue to read the more in debt explanation from the Wikipedia's article.
Source: AFPMB, Wikipedia