I pick sharks. Dolphins are friendly to humans and cuter, but they also are fierce predators too. But I choose sharks because they are long lasting predators of the ocean that have lasted a long time, having existed longer than mammals. Most sharks aren't harmful to humans either, and they have interesting physiology, being made of cartridge. Plus they are very diverse in species, ranging from peaceful filter feeders to fearsome top predators like the Great White Shark.
Octopuses or Jellyfish