Cole Porter, hands down.
Johnny Mercer, and George and Ira Gershwin penned some pretty memorable songs, too.
In more recent history: Joni Mitchell would be up there, as would Dylan. Marvin Gaye. Lennon and McCartney we can take as given (definitely not Lennon on his own, definitely not McCartney on his own, sorry). Short career, but by God Paddy McAloon penned some memorable tunes. Not always wild about his lyrics.
Just for MES: Morrissey composed a number of good songs, although I do get weary of that voice like cattle lowing in the meadow…
Shout-out for most ridiculously underrated songsmiths in rock history: Colin Moulding and Andy Partridge.
Ray Davies? Obviously.
Have I gone past six? Oops.