[Jim Moonan]

For plainspoken profundity, Frost is a master at opening lines:
Two roads diverged in a yellow wood (from The Road Not Taken)
Something there is that doesn’t love a wall (from Mending Wall)

And this ending line, from Frost’s Birches, always leaves me dumbfounded:
One could do worse than be a swinger of birches.

[Julie Steiner]

There are more nominees at this old thread.

[Brian Watson]

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The firefly wakens: waken thou with me.

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[Brian Watson]

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Out, out, brief candle.

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[Brian Watson]

How at last the sand so gently grinds the glass
-Merrill

He's talking about an hourglass, which over the course of years runs faster as the sand erodes the neck between the two lobes. An apt metaphor for the way time seems to speed up as we grow older. I like the delicate assonance/consonance, and the subtle metronomic effect (in comparison to the more thumping "When I do count the clock that tells the time"). And it reminds me of a passage from Herbert's Church Monuments:

That flesh is but the glass, which holds the dust
That measures all our time; which also shall
Be crumbled into dust.

[Roger Slater]

That reminds me of Shelley's great line:

Life, like a dome of many-colored glass,
Stains the white radiance of Eternity