What is the most important metaphor in The Way to Rainy Mountain?

Because Momaday does not actually name the "most important metaphor" for his readers, we are left to decide for ourselves. Your question, then, is an opinion question.  In my opinion, the most important metaphor in The Way to Rainy Mountain can be found in the significance of the title itself. Rainy Mountain becomes a metaphor for the myth, history, and personal experience of the Kiowa tribe of Native Americans.

We can use a few pertinent...

Because Momaday does not actually name the "most important metaphor" for his readers, we are left to decide for ourselves. Your question, then, is an opinion question.  In my opinion, the most important metaphor in The Way to Rainy Mountain can be found in the significance of the title itself. Rainy Mountain becomes a metaphor for the myth, history, and personal experience of the Kiowa tribe of Native Americans.

We can use a few pertinent quotations to further explain the above idea in The Way to Rainy Mountain:

A single knoll rises out of the plain in Oklahoma, north and west of the Wichita Range. For my people, the Kiowas, it is an old landmark, and they gave it the name Rainy Mountain.

The metaphor of Rainy Mountain is furthered by Momaday's description of "the way" to this Kiowa landmark. There is an explanation given for that as well:

I returned to Rainy Mountain in July. My grandmother had died in the spring, and I wanted to be at her grave.

If you put these two quotations together with the organization of the book, the metaphor becomes even more clear. The book contains three main parts: "The Setting Out," "The Going On," and "The Closing In." These main  parts are divided into twenty-four numbered sections written in three separate voices: one about myth, one about history, and one about personal experience. 

Every single part, section, and voice leads directly back to the main metaphor: Momaday's "way" back to "Rainy Mountain" as Momaday's research and discovery of his own Kiowa heritage and religion. Momaday's metaphor becomes complete as he learns about the Kiowa myths, researches the dates of Kiowa history, and listens to his own grandmother, Aho, as she tells her own personal stories about the Kiowa tribe.