I thought of Carol King, but the volume of her songwriting and the length of time she wrote new songs just doesn't measure up to her one great album.
There are lots of great bands with great songs but no one person in that group that stands out. Zeppelin has great songs but much of what they did were old blues songs reinterpreted. Allman Brothers the same. Aerosmith, Yes, Genesis, Moody Blues, Americ, Stones and many others were performance bands and collaborative writers but not necessarily one song writer. Current country and pop artists don't write their own stuff much either.
Ian Anderson from Jethro Tull could be on the list somewhere.