In all likelihood, the best songwriter is some guy unheard-of outside his home town, whose music none of us will ever get to hear. Sorry if this sounds unduly pessimistic, but it's a fact that (due to the vagaries of chance, luck and capitalism) many of the best songs never even get recorded, let alone commercially released. That said, of all the songwriters who've had the good fortune to be widely heard of; these are in my view deserving of particular attention:-
Bob Dylan
Leadbelly
Bert Jansch
Richard Thompson
Cyril Tawney
Bill Monroe
Morrissey (yeah, seriously!)
Nick Drake
Jackson C. Frank