"Reapers. Names given to us by the Protheans to give voice to their destruction. In the end, what they chose to call us is irrelevant. We simply...are."
Best line of Mass Effect, superb writing and whoever did the voice acting and sound effects for Sovereign nailed it epically. They could easily have gone with a cheesy "We are reapers for we reap the galaxy of sentient life!" or something, but their trivializing of the name and complete disregard for its meaning really give the reapers an enigmatic malevolence. That they don't take heed of any conventional line of morality, or even thought, that organic life has.
"So wake up, Dr. Freeman. Wake up and smell the ashes."
Also loved that opening line from the G-Man in Half-Life 2, did a nice job of setting the tone for the rest of the game.
"There are few of us now. We know we will soon be gone. And so we see our fate as others will see it. There will be little legacy, little left of what we once were. Our accomplishments, our technology...if ever they were seen again would spawn none of the awe that filled our conquests."
One of the excellent cinematics from Freespace: The Great War, an under-appreciated gem of a game. Wording might not be exact, I'm going off memory, but that was the gist of it. I'll look up the cinematics on youtube when I get a chance.