Gee, there's been so many.
For me, nobody tops Bob Dylan. But John Lennon/Paul McCartney is almost as great, as is Elton John/Bernie Taupin. Of the more recent writers, I love Norah Jones' work.
f you want to go back a way, it's hard to overlook Jerry Leiber and Mike Stoller, such classics as Ruby Baby, There Goes My Baby, Dance with Me and Stand by Me for the Drifters and Ben E. King....Jailhouse Rock, Don't, and King Creole for Elvis....Yakety Yak, Charlie Brown, and Along Came Jones for the Coaster, and so many others.
Another awesome writer who often doesn't get mentioned is Carole King. If you look up her writing credits, she wrote an amazing number of huge hits for other artists years before she hit the big time with her Tapestry album in the early 70's....she wrote Chains by the Cookies, Some Kind of Wonderful by the Drifters, The Loco-Motion by Little Eva (and Grand Funk, and others), One Fine Day by the Chiffons, Pleasant Valley Sunday by the Monkees, Up on the Roof by the Drifters, and so many more.
And if you want to go way, way, way back, it's hard to beat the works of either Irving Berlin or George and Ira Gershwin.
But for me, Dylan will always be #1.