What is the least common jellyfish?

um... im doing a oceanography project for science in school. we are making an ocean wall and one of my bethic animals is a jellyfish. I know that there are different kinds of jellyfish and i want to research the least common jellyfish, for a change. A lot of people likes to do the most common because its easier to get information. Iwant to show people that there are other kinds of animals that people don't care about cause they are not common.well, do you know the least common jellyfish? if you do plz tell me. THANX!!!