Anime obsessives are going to be pretty high up. I've actually bumped sci-fi obsessives from my list because these people do the same but so much worse (to the extent that sci-fi types are kind of a romantic ideal nowadays: I saw a man carrying a fullsize replica of K-9 through Victoria station on Saturday and could only smile sweetly at him). 98.2% of all webtraffic on wikipedia is generated by people arguing about editing the page for Naruto.
Music obsessives? It's a bit wide to define. Maybe split it by genre. Proper pop/chart music obsessives are pretty awful.
My vote is for videogame obsessives. Some real mentals here. My favourites are the pseuds. Spotted this guy on a forum a little while ago where he linked to his own site.
My favourite bit is the list of books that you have to read in order to be able to understand his own upcoming book.
Nietzsche (Note: The best English translations are by Walter Kaufmann and R.J. Hollingdale)
The Gay Science
Beyond Good and Evil
Twilight of the Idols
Thus Spoke Zarathustra
On the Genealogy of Morals
The Birth of Tragedy
The Will to Power
Simulacra and Simulation
In the Shadow of the Silent Majorities
The Consumer Society
Symbolic Exchange and Death
The Transparency of Evil
The Conspiracy of Art
The Intelligence of Evil or the Lucidity Pact