You could probably make a compelling case for all of them embodying what it means to be American. Honestly, each one features something about them that is uniquely American or holds within their thoughts what it means to be American. I think that the "textbook" answer would be that Walt Whitman most strongly identified his work as representing that which is to be American. Whitman was eager to represent t how the nation's soul is to be held within his work. When one reads, "I Hear America Singing," for instance, one hears the praising of American heterogeneity, the eclectic nature of what it means to be American, and the notion that America, as a nation, is something that is to be echoed through Whitman's own subjectivity. Whitman was a devout believer in the idea of democratic vistas being wide as possible and embodying the hope and promise of American identity that helped to give voice to the young nation. In the end, of the four poets mentioned, I think that Whitman's spirit, passion for American democracy, and the exaltation of what the democratic experiment meant for his soul and his nation makes him a spokesperson for the American experience.