I could - and do - agree with very many of the names put forward in this thread. Heading my list would be icons such as Dylan, Bowie, Lennon/McCartney (Lennon for his unashamed and frequent baring of his soul and McCartney for his remarkable talent to create stories with real characters (lets not dwell on Dance Tonight!)).
I just wanted to suggest a songwriter whose name hasn't appeared in this list so far..... Billy Joel. It's not that I'm a huge fan of his music but lyrics to songs such as 'Always a Woman' mark him out from the horde and IMO he merits a mention here.