My favorite short poem, though hardly an original choice, is Frost's "Fire and Ice." Hard to beat. At least three dozen Dickinson poems would also be in the running.
Though not a "poem" unto itself, I've always thought of these lines from Shelley's "Adonais" as being able to stand on their own quite beautifully:
Life, like a dome of many-colour'd glass,
Stains the white radiance of Eternity
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