Jellyfish

The lion's mane jellyfish, Cyanea capillata, have long been cited as the largest known jellyfish, and are arguably the longest animal in the world, with fine, thread-like tentacles up to 36.5 m (120 feet) long (though most are nowhere near that large). They have a painful, but rarely fatal, sting.
The increasingly common giant Nomura's jellyfish, Nemopilema nomurai, found some, but not all years in the waters of Japan, Korea and China in summer and autumn is probably a much better candidate for "largest jellyfish", since the largest Nomura's jellyfish in late autumn can reach 200 cm (79 inches) in bell (body) diameter and about 200 kg (440 lbs) in weight, with average specimens frequently reaching 90 cm (35 inches) in bell diameter and about 150 kg (330 lbs) in weight. The large bell mass of the giant Nomura's jellyfish can dwarf a diver and is nearly always much greater than the occasionally-up-to-100 cm bell diameter Lion's Mane.
The rarely-encountered deep-sea jellyfish Stygiomedusa gigantea is another solid candidate for "largest jellyfish", with its 100 cm wide, and thick, massive bell and four thick, "paddle-like" oral arms extending up to 600 cm in length, very different than the typical fine, threadlike tentacles that rim the umbrella of more-typical-looking jellyfish, including the Lion's Mane.