Jellyfish account for more than 80 known deaths since 1883. The box jellyfish (pictured) was responsible for 79 deaths, and Irukandji the other two. Both jellyfish have tentacles that are generally invisible, can entangle you and have millions of harpoons that inject a lot of venom at once. The box jellyfish in particular is the only one on this list that can kill in minutes. Ergo, highly dangerous.
Photo Credit: Nick Rains / Australian Geographic