Active writers (so no Neil Gaiman, Alan Moore, Frank Miller, or China Mieville sadly).
Based on a combination of style, skill, and my own preference (some writers I think are better may be lower than a writer I just simply like). Oh, and also personality can have an influence for some.
As w/ all my lists, it's a mix of subjectivity and objectivity. The list isn't purely "favorite" nor purely "best", but considers both, and considers all work I've read regardless of its age (but the work's contemporariness may play a factor).
I'd love to do one for artists, but...I just love so many so equally!