Interesting topic. Many times we tend to give credit to the singer and forget that lyrics had to be written for any song. Many can name songs/artists, but the songwriter doesn't always get the credit he deserves it seems.
Most of my favorites have been mentioned; Lennon/McCartney, Paul Simon being two of them.
Billy Joel, who I think, has written most of his songs, has such diversity in his music style, and is a genius at successfully showing several different sounds. For instance; "It's Still Rock & Roll To Me" has sort of a 50's type lyrics/sound; Uptown Girl reminds me of a Four Season's number, and Anthony's Song is just plain different in sound. Of course don't forget "We Didn't Start The Fire" which has yet another sound/feel to it. I may not care for every single song Joel has written but I do think he is an extremely talented performer/song writer.
TIS