If you're talking composers than Jeff Lynne and his work with ELO has to be rated highly.
We have an endless history of great songwriters in contemporary music, decade after decade.
Bowie was an incredible writer, could paint a picture with lyrics or the music.
The Doors wrote some very interesting stuff, so too Jagger and Richards.
Simon and Garfunkel had a great partnership.
The Bee Gees were absolute masters at songwriting.
In country music we have been blessed with Dolly Parton, Kris Kristofferson, Willie Nelson, Hank Williams etc.
Obviously Sir Paul and Lennon were at a level that influenced music, fashion and culture, and though he doesn't get a lot of credit as a writer Harrison was not far behind them.
As far as Aussie songwriters go INXS were great writers of commercial music and managed to have a very unique sound. The Finn brothers know how to write great songs as well.
The Easybeats were awesome as writers.
How lucky are we to have great music to listen to across so many decades.