While I know no poem in the language greater than Gerard Manley Hopkins's "The Wreck of the Deutschland," and confess a supremely warm spot for Joyce Kilmer's cozy but forgotten "The 12:45," a favorite is this sonnet by Millay from her collection Fatal Interview.
Now by the path I climbed, I journey back. The oaks have grown; I have been long away. Taking with me your memory and your lack I now descend into a milder day; Stripped of your love, unburdened of my hope, Descend the path I mounted from the plain; Yet steeper than I fancied seems the slope And stonier, now that I go down again. Warm falls the dusk; the clanking of the bell Faintly ascends upon this heavier air; I do recall those grassy pastures well: In early spring they drove the cattle there. And close at hand should be a shelter, too, From which the mountain peaks are not in view.