I have two. The most Obvious is John Denver. The other, NOT so obvious, and most forgotten is Jim Croce. These two guys wrote songs about stuff in thier lives. Real happenings. We've all broken up with someone because of a best friend. That's what Croce's "Operator" is about. I could go on all day, but I won't.
Oh yeah - Aaron Stainthorpe from My Dying Bride is really good. I don't know if Aaron writes along with the music too but his lyrics are incredible and very deep (and if he doesn't contribute to writing the music it's no big deal because the music is equally incredible). Alanis Morissette is great, definitely my most favourite modern pop music songwriter, maybe alongside Tori Amos but Tori always has only a few songs that really touch me per album and the rest doesn't really get to me so much... I think Satyr and Frost from Satyricon are among the best black metal songwriters, they sound fresh and varied without losing that cold feeling only Norwegian black metal possesses. I really love their first three albums to death. Nergal from Behemoth succesfully transformed his band from decent black metal to brilliant and, equally important, original death metal, which deserves some credit in my opinion. Jon Schaffer from Iced Earth is a nationalist bastard, and their latest album didn't quite have *it* to me, but he has written brilliant songs.