For the Romantics, nature and imagination are the most important elements.
Discuss this with references to the poems "Rime of the Ancient Mariner," "Tintern Abbey," "Ode to the West Wind," and "To Autumn."
I guess I have to diverge a bit from the phrasing of the question. I think that Romantics were not able to separate the embrace of individuality, the love of nature, moral structure, exploration of the imaginative powers of the self, establishment of artistic autonomy, and the exploration of the subjective in the world into a hierarchical fashion. They all seemed to converge into forming the Romantic creed of beliefs and values. Even in the poems featured, I find it difficult to establish one idea as more dominant than other elements. Certainly, Coleridge's poem features nature and a sense of moral imagination being expanded, but there is an equal emphasis on being able to identify a structure that governs human behavior in the killing of animals. This is as prevalent as nature and imagination. Wordsworth's ode to maturation is just as dependent on nature and imagination as it is on the lauding of the personal with "the child as the father of the man." The works from Keats and Shelley are just as concerned with the role of the artist and how the artist conceives the universe and his/ her individual setting just as powerfully as imagination and nature galvanizes them.