How can I use the work of WB Yeats to argue that a poem is the perfect form of expression?

To answer your question, I will refer to two of Yeats's best-known poems, "An Irish Airman Foresees his Death," and "Sailing to Byzantium."
In the first of these, it is easy to see that it would be impossible to put forth the poem's theme so forcefully in prose. Yeats has the speaker express what is essentially the heroic attitude toward death. The conciseness of poetic form is what enables him to do this so effectively. In sixteen lines, Yeats is able to include, as well, corollary ideas that would require paragraphs of prose in which the reader could easily lose track of the thought, and in which the various points would not be made as memorably.
Perhaps the most moving lines of this poem are: "Those that I fight I do not hate, / Those that I guard I do not love." The speaker feels alienated from both his own country, and from the cause for which he is fighting, but he fights anyway. We do not know if this is because of the religious divisions within Ireland, but the fact that this is an open question makes the poignancy of the thought all the more striking. "No likely end would bring them loss, / Or leave them happier than before," is, as well, a not fully explained thought in which the implication is, perhaps, the pessimistic conclusion that regardless of who wins the war and rules Ireland, the result will be the same for the oppressed people. All of this needs to be seen in the context of the centuries of rule in Ireland by Britain, which, soon after World War I, would come to an end for most of the island. Most Irish people had little reason to have patriotic feelings about the Allied cause against Germany in the Great War.
The overall effect is, as with much poetry, the paradox that it expresses ideas more forcefully than prose, but does not explicitly clarify them, leaving much to the reader's interpretation.
Yeats's "Sailing to Byzantium" is a much more complex work, but the overall idea, which I would argue is the human soul's wish to be relieved of physical constraints, is stated in such a way that, in a non-verse form, it would possibly sound like a mere truism, as opposed to an original thought. The arresting opening, "That is no country for old men," is a signal that something of special value, perhaps contradicting conventional thought, is going to be presented here. One can't overstate the power that rhyme gives this, and indeed, most poetry. The closing couplet of the first stanza, "Caught in that sensual music all neglect / Monuments of unageing intellect," is a deliberately ambiguous thought, given power by the rhyme. To present the unexpected is a hallmark of Yeats's approach, illustrated throughout the poem but most significantly in the conclusion of the second stanza: "And therefore have I sailed the seas and come / To the holy city of Byzantium."
Here, too, is the expression of an idea that would be impossible in prose. What is the significance of Byzantium, and why does Yeats choose it as his goal? There is no explicit explanation, and again, this incompleteness gives the basic idea at the root of the poem a greater power than it would otherwise have. Byzantium is invoked as a symbol of holiness, and because it stood midway between West and East, as a spiritual bond for mankind. It is an emblem of that extra-physical realm in which the speaker wishes to have as his abode. As a symbol, a metaphor, it touches the reader more directly than a detailed and prosaic discourse on the subject would have the power to do.
Poets have always recognized that ideas are more forcefully communicated, and are made more memorable, by the elements of poetry: meter, rhyme, imaginative language, simile and metaphor, and so on. These two works by Yeats illustrate this fact at least as fully as any other poems in the English language.