Shelly's "A Lament" is a poem of grief for the loss of life's prime. As such, the most powerful quotes would be the portions that elucidate the theme, the speaker's feelings, and the affects of the loss. Therefore, one of the first quotes to consider is:

When will return the glory of your prime?

This line delineates the subject of the poem and makes it clear that "On whose last steps I climb" refers not to Time's final steps, as in the last and final steps, but to recent steps, as in recently taken, because he has stood there before. This quote is powerful because it is here that you realize the speaker is lamenting his lost prime of life, his youth, which encompasses all the creative productivity that accompanied his prime.

The speaker's feelings (and in this poem, many assume the poetic speaker is Shelley himself) are elucidated in this line:

Trembling at that where I had stood before;

We know he is climbing upward on final steps. To learn now that he is trembling as he stands in a familiar spot calls forth the force and depth of his emotion. The next stanza sheds light on his feelings when he states that "A joy has taken flight." Both of these are powerful because they reveal the inner psychology of the speaker. This is also true of the following:

Move my faint heart with grief, but with delight
No more

The twice repeated refrain, "No more -Oh, never more!" might arguably be called the most powerful because the refrain adds the quality of a hopeless and final lament over an irretrievable loss:

When will return the glory of your prime?
No more -Oh, never more!