For me it’s the last sentence of Lincoln’s First Inaugural. Let me quote the whole paragraph for context.
The sentence consists of two memorable phrases “mystic chords of memory” and “better angels of our nature” joined by a wonderful double-metaphor: the chords both stretch in space and “swell the chorus” with music. The sentence has a lovely balance and rhythm but is also rich with historical resonance; it was uttered at the moment of supreme crisis in US history. It’s prophetic but tinged with tragedy and irony: Lincoln’s own assassination and legend were crucial to the national reconciliation he predicted.
There have been other important sentences in world history, and some just as beautiful, but I can’t think of anything that combines beauty and importance in quite the same way.