Depends on if the question really means "what do you read most of" or "if you were stranded alone on a deserted island forever, and could only have one category of books, what would it be?" (With, of course, the caveat that "How To Build A Boat" is not allowed.)
Read most of: Erotica. Mostly fanfic slash.
If I had to read only one forever: Science fiction.
There's more free, well-written slash than free, well-written science fiction on the web.