I'd have to agree with a number of people above (particularly
those that said there's no one "best song writer"; depends
on my mood and what genre I'm looking for as to who is currently
my favourite). But there's one who hasn't been mentioned above,
who, if you look at the acclaim they get through other
folk performers covering their songs (not counting such pop artists
as the Beatles and Dylan), should by all rights be on the list
up there, along with Seeger, McColl, and Malvina Reynolds:

Bruce "Utah" Phillips

Just MHO.

Cheers,