I don't think that there is a "perfect line" that easily and uncontestedly (is that a word?) retains its perfection outside its own context. It's a complicated issue to say it that way, however, because it must be remembered that not every poem has a perfect line hidden in it. But context isn't always the poem (sometimes can't it be the subject matter?) I think that there are lines that retain something of what I consider 'perfection' outside of their own poems.

In short, it all really depends on what the reader is looking for in a poem.

For me, perfect lines are characterized by delicate, careful eloquence, and a rushing feeling in your chest, like falling from a great height. One example comes from "Scheherezade" by Richard Siken:

Tell me how all this, and love too, will ruin us.
These, our bodies, possessed by light.
Tell me we’ll never get used to it.


The spacing gets a bit messed up in transit, but there it is. It's about, as one review of the collection said, "nerve wracked love."