Each reader is likely to interpret N. Scott Momaday’s book in a different way, but one central metaphor one could look at is the grandmother. While Momaday incorporates the organizing principles of the hero within the existence of a quest and the adventures of travel, the grandmother is distinct because she represents family, the larger family-like Kiowa community, creativity, and the cycle of life. Grandmother and grandfather are synonyms for ancestors as well as terms that designate relationships between specific individuals of different generations.

Trying to figure out the significance of his grandmother’s life, actions, and stories is the catalyst that takes Momaday on the journey into his own family’s past and into Kiowa culture. The experiences she remembers—such as the Sun Dance that was later banned—and her creative reconfigurations of the mythic past are both significant contributions that she makes and that stand for Kiowa creativity. The severe rupture that came with her death represents Momaday's own break from his culture. The fact that he not only reconnects with it, both through physically traveling to important sites in Kiowa territory and through embarking on creative projects, indicates his position in the phase of rebirth that follows death.

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.