This is not a hierarchy but simply a list of the many species of genus otakas
1. The classic otaku (Otakas antiquitus)
It conforms to all the classic Japanese stereotypes. It is always male, either grotesquely fat or grotesquely skinny (to the point where you believe him to be an escaped prisoner from a concentration camp), and is usually dressed in a rather unkept manner. They are known to enjoy all forms of hentai and thus wear thick glasses to fully experience it. Social skills are deplorable, even to other members of the herd. They are thought to have originated in Scotland as they seem to wear plaid an awful lot. Reproductive habits are unknown at this point. Despite being the oldest known species of otaku, much research is needed.
2. The casual otaku (Otakas normalius)
The largest species of otaku currently. This author is part of this species. Many casuals enjoy a wide variety of anime while some may only watch the mainstream stuff. A small fraction are known to enjoy hentai, but this information is only a theory. Mating habits are quite erratic, from regular to irregular. Casual female fans are considered to be entirely different from fangirls
3. The fangirl (Otakas feminus)
the second largest species of otaku. Unlike most otaku, fangirls form packs usually headed by an alpha female who has demonstrated great skill in fan fic writing, cosplaying, or pokemon skill. According to research done by the author, they apparently reproduce asexually, as many would not date the author or many other casuals, this has led many to classify them as a separate species. Their intensity is most fierce and has created a sort of tribal society. Yaoi is the universal religion of fangirls and primary romantic stimulus.
4. The pygmy otaku (Otakas minimus)
Otherwise known as the 8 year-old obsessed with Naruto. The pygmy draws crappy renditions of characters during class and memorizes each episode to the letter. Pygmies are also attracted to Dragonball Z and other mainstream capitalized series. They apparently reproduce using spores that infect casuals into liking the same crap as they do. Hentai habits are none as their parents don't let them. Some pygmies may form colonies.
This research is incomplete and this author would appreciate any information provided by an independent source.