In the poem, "The Second Coming," which single word (use of diction) do you see as the most significant as to the poem's theme or argument?

From one perspective, the word "gyre" is the most significant word in William Butler Yeats's poem "Second Coming." If one chooses to interpret the poem based on the poet's deliberate choice of words, then gyre is the most loaded term in the work. Yeats created, with the help of his wife's automatic writing that passed on ideas from supposedly ethereal "teachers," a complicated philosophy/religion by which he interpreted history. Gyres were central to this philosophy. Although many who are acquainted with Christianity are tempted to equate the "second coming" of the poem with the second coming of Jesus Christ, that is not at all what this poem is about. Instead, in Yeats's view, the 2,000-year gyre (era) that began at the birth of Christ was reaching its widest point—its end point—and would soon spin off a new 2,000-year gyre. The next period of history would be characterized by a "rough beast" as mentioned in the final question of the poem.
However, if one chooses to pass over the complexities of Yeats's theories of gyres and interpret the poem based on the imagery and wording that one can understand without any esoteric knowledge, then "pitiless" presents itself as the most important single word. The imagery of the coming age is a sphinx-like creature with a "pitiless" gaze. This image, and the idea of an age bereft of pity, or compassion, represents the modernist rejection of a benevolent God in favor of a malevolent Fate. Yeats was far from alone in his understanding of a force that arranges world and personal affairs not out of love, like the Christian and Jewish God, but out of randomness at best and malice at worst. For example, Thomas Harding's poem "The Convergence of the Twain" speaks of the Immanent Will—a malicious Fate—that planned the "welding" of the Titanic and the iceberg.
The word pitiless, then, aptly represents Yeats's argument that the coming era would be best understood as orchestrated not by a loving God for the good of his creation, but by an uncaring force that would unleash undreamed-of terrors on the upcoming generation.
https://blogs.haverford.edu/celticfringe/2014/02/25/can-the-centre-hold-yeats-and-gyres/