Vimes hardboiled egg speech from night watch, would be up there. But Death's speech to susan on the nature of his job in soul music is the one that sticks out the most.
"But most people are rather stupid and waste their lives. Have you not seen that? Have you not looked down from the horse at a city and thought how much it resembled an ant heap, full of blind creatures who think their mundane little world is real? You see the lighted windows and what you want to think is that there may be many interesting stories behind them, but what you know is that really there are just dull, dull souls, mere consumers of food, who think their instincts are emotions and their tiny lives of more account than a whisper of wind."