What are considered the three most poisonous jellyfish?

The three most poisonous jellyfish are the box jellyfish (Chironex fleckeri), the Irukandji jellyfish (Carukia barnesi), and the Portuguese man o' war (Physalia physalis). The box jellyfish is known for its potent venom that can cause heart failure and death within minutes. The Irukandji jellyfish, though small, can cause severe pain and potentially life-threatening symptoms known as Irukandji syndrome. The Portuguese man o' war, while not a true jellyfish, possesses a venomous tentacle that can deliver painful stings and allergic reactions.