Obviously it's impossible to select a perfect line from a poem, but it's a fun and interesting test of one's emotional sensibilities and technical sensitivities. Mine are not arbitrary, though they're lucky to be picked ahead of one or two other contenders.

If I had to pick a perfect line from a poem, I'd go either for Shakespeare's beautifully cantilevered line, "I wasted time and now doth time waste me," spoken by Richard II, or instead Marlowe's exquisite iambs in Faustus's melody of damnation: "Was this the face that launch'd a thousand ships / And burnt the topless towers of Ilium?"

These lines, methinks, can never be improved upon! So, come on, what do you think is poetry's "perfect line"?

I feel I have to submit this line from e.e. cummings, even though I suspect it's stuck in my mind (mainly) due to it being quoted in Woody Allen's Hannah and Her Sisters:

nobody, not even the rain, has such small hands

I've absoulutey no idea in what is or makes a 'perfect' line of poety. But a fave line, a part of which I always seem to scratch whenever doodling, comes from Yeats' Byzantium. It's the third line in the following quote:

An aged man is but a paltry thing,
A tattered coat upon a stick, unless
Soul clap its hands and sing, and louder sing
For every tatter in its mortal dress,

On the subject of 'perfection', Latin scholars look for the 'golden line' - which goes adjective-noun-verb-noun-adjective, where the first adj agrees with the second noun, and the first noun with the second adjective. For example, in Catullus poem 64, the story of Theseus and Ariadne, in which Theseus - the cad - abandons Ariadne on an island and rows off without her, "irrita ventosae linquens promissa procellae" ("leaving the useless promises of a windy storm (behind him)") ...

I can only offer my three most favourite (albeit a tad melancholic) lines of poetry, which have haunted me since I first read them:

The heart shuts
The sea slides back
The mirrors are sheeted.
(Sylvia Plath, 'Contusion', 1963)

Thank you very much, sheri. Lovely lines, all of them...