Harold Arlen, without question. Some of the others mentioned here are great-- Guettel, JRB-- but there's Arlen and then there are the others.
I also hold the theory that the reason why, unlike lyricists who wrote with multiple partners and didn't become true household names, like Dorothy Fields and Yip Harburg, Johnny Mercer became just that because of his wonderful singing career. He was a chart-topping singer in the 40s and 50s, and had a wonderful way of singing and styling a song. And it made him very famous.