Walt Whitman is, indisputably, America's poet. He is vast, large, contradictory (Do I contradict myself?/Very well then I contradict myself/(I am large, I contain multitudes)), beautiful and loose and American to the core!
His greatest poem is, in my opinion, "Song of Myself." This is far from a controversial opinion, and for good reason; the eighty-odd page long poem is an astounding epic--albeit, an unusual one, but a monumental achievement of literature. It is Whitman as Everyman, Whitman as you, as me, as all other mortals from China to Peru. I quote his beautiful closing stanzas:
"I bequeath myself to the dirt to grow from the grass I
If you want me again look for me under your bootsoles.
You will hardly know who I am or what I mean,
But I shall be good health to you nevertheless,
And filter and fibre your blood.
Failing to fetch me at first keep encouraged,
Missing me one place search another,
I stop some where waiting for you"
Such beauty in verse, especially free verse, is scarcely found, and, when found, must be cherished. There is a reason almost all poets after him--and not just poets in the English language, either (Borges, for example, aspired to be the "Whitman of Argentina")--have been influenced by him more so than any other poet besides perhaps Shakespeare and Milton.
Nor is "Song of Myself" his only great poem, though it surely be his greatest. His elegy for Abraham Lincoln, "When Lilacs Last in the Dooryard Bloom'd" is monumental (the great critic Harold Bloom declares it Whitman's finest poem, and thus the greatest of all American poems--I dissent, but uphold its marvel nonetheless), as is almost all of his wonderful corpus of poetry. Whitman is remarkable; he is inescapable; he is beautiful. Read him, and thou shalt be infinitely rewarded.