I can't see how you can include Anon or trad, as these are not songwriters but attributes applied to certain songs where the original writer is unknown and, in any case one writer didn't write all of these!

I don't like using "Best" as it is only an opinion and can't be measured. You could however ask about most prolific or successful in the folk music genre (if there is such a thing) I would think about that the songs that I hear again and again at folk clubs not including traditional and the first three most prolific that spring to mind are Bob Dylan, Eric Bogle and Ewan McColl