I could never pick just one...being an English major with a french lit minor prevents it. But, I am always fond of:
I WILL arise and go now, and go to Innisfree,
And a small cabin build there, of clay and wattles made;
Nine bean rows will I have there, a hive for the honey bee,
And live alone in the bee-loud glade.
And I shall have some peace there, for peace comes dropping slow,
Dropping from the veils of the morning to where the cricket sings;
There midnight's all a glimmer, and noon a purple glow,
And evening full of the linnet's wings.
I will arise and go now, for always night and day
I hear lake water lapping with low sounds by the shore;
While I stand on the roadway, or on the pavements gray,
I hear it in the deep heart's core.
Yeats' Lake Isle of Innisfree.
Poetry shifts for me. I think a "favorite" is something that suits your mood at the time. Some nights I might particularly enjoy something beatific by Ginsberg or Ferlinghetti...other times something by Poe or something like Dover Beach by Arnold.
A poem that is just plain great to HEAR is Ferlinghetti's "Dove Sta Amore".
Dove sta amore
Where lies love
Dove sta amore
Here lies love
The ring dove love
In lyrical delight
Hear love's hillsong
Love's true willsong
Love's low plainsong
Too sweet painsong
In passages of night
Dove sta amore
Here lies love
The ring dove love
Dove sta amore
Here lies love
But you know what, it's pretty hard to beat reading Poe's "The Raven." The language he uses is just absolutely masterful.