The most poisonous fish in the world is probably the puffer fish, which has the most deadly venom of any vertebrate apart from the golden poison frog. In particular, the liver is extremely poisonous. Nevertheless, other parts of the puffer fish are safe to eat if prepared carefully, and in Japan it is offered as a delicacy by specially trained chefs.
The stone fish has a venom almost as deadly as the puffer fish, and in fact is even more dangerous because you don't have to eat the stone fish in order to be killed by it; they have spines that they stick out when threatened, and swimmers can step on these by mistake.
The piranha fish, on the other hand, in spite of its fearsome reputation, is less dangerous to humans although they are aggressive and will bite when disturbed. However, they are not venomous. And the most famous deadly fish of all, the great white shark, also doesn't attack humans as often as is supposed, though of course they are very dangerous when they do.