Imagination in Wordsworth's "Tintern Abbey" is that which connects to nature and transforms the speaker, and creates in the speaker a sublime mood. The poem is more about the effect the surrounding scene had on the speaker five years before when he first saw it, than it is about the surrounding scene in the poem's present. While contemplating the scene after he experienced it earlier, he was led to acts of kindness and to a heightened mood and sense of the awe of nature. The imagination is something that is improved and trained, and something that connects to the transcendent, that which is beyond human understanding.
In Coleridge's "Ancient Mariner," imagination explodes into the supernatural. The supernatural connects to nature to avenge a slight against nature--the senseless killing of the albatross. Coleridge sought to create a work of perfect imagination, and this poem is such an attempt.
Shelley in "West Wind" imagines his poetry as doing to the world of thought and imagination and poetry what the winds do to nature. The West Wind transforms autumn to winter, and its sister transforms winter to spring in the spring. Shelley imagines his poetry doing the same. He uses personification and metaphor to show the wind's transforming power, then pleads with the West Wind to help him do the same with his poetry.
For quotes you can see the early to middle stanzas of Wordsworth, parts II and III, particularly, in Coleridge, and parts I and V in Shelley.