After editing down the original question, I think that it should be noted that Coleridge's Romantic tendencies are evident in so much of his work. Being on the "ground floor" of the movement along with Wordsworth meant being an active contributor towards defining the movement and what represented Romanticism. I think that Coleridge is at this zenith in "Kubla Khan." The very nature of the Romantic essence is on display in this poem. The notion of dream and visions being central to the Romantic ideal is evident in the poem. It is one in which subjectivity becomes essential to what Romanticism is meant to embody. Coleridge's poem seeks to embody the Romantic movement in its focus on the subjective experience being broadened out to form a universality within it. Another Romantic element that is on display in the poem is the idea that Coleridge's work expands on the notion of what can be as opposed to what is. "Kubla Khan's" description of Xanadu is a realm that is meant to be a polar opposite of this reality, an element that seeks to establish the Romantic imagination:
A savage place! as holy and enchanted/ As e’er beneath a waning moon was haunted/ By woman wailing for her demon-lover!
Romanticism was driven by the imagination, a spirit that unleashed could be profound in the transformation of the individual and the social order. Coleridge understood this and in constructing his poem placed such Romantic ideas at the center-point of its being. It is in such an idea where I think that a great argument can be made for "Kubla Khan" being the poem where Coleridge's Romanticism is best on display.