In country, Ira Louvin. His lyrics were remarkable, far advanced for their time (which is why Louvin Brothers songs have stood the test of time).
In rock, I have two. My favorite living songwriter is John Hiatt. He has such a wonderful way with words and can write deeply personal songs that make you feel his pain then turn around and make you laugh with the next song. My favorite rock songwriter is Warren Zevon. He could be funny, desperate, introspective, sarcastic, optimistic, and romantic -- occasionally in the same song.