the best - I love the Yeats one quoted by painful_ginsu, and also Christopher Marlowe's "Passionate Shepherd to His Love" albeit a little too oft-quoted.

my personal favorite (by no means "best," but best to me) is one of my own, "child":

primordial child
you bring the past alive for me
as you run along the edge
of the Earth and sea
So newly born of them,
you radiate their wisdom
and stay their momentum
with your every nimble step
You connect me to the ancient
and the timeless
And fill me with the
voices of the ages

Your smile carries me
to times as yet unknown
Like the Cheshire Cat's,
holding
as the background dims and changes
And I see the smile of a man
falling into himself
as his bride glides
on a cloud of love,
to stand at his side;
The smile of a father
seeing his child born -
Both crying,
the circle complete

The voices chorus
as the smile tips upwards
and you search my eyes
You reach for me -
lift my heart as I lift your body
You've come for a kiss
a hug
some milk
I fold you in
and close my eyes
to hear the hush of the waves
The weight of your body in my arms
tells me you are flesh
But I know
you are made of love
and all there ever was