David Tufte states that it's intellectually rewarding to look at sets of quotes that friends and colleagues collect. I agree.

But I haven't compiled my list yet. However, I can recommend Thomas Sowell's list and I can offer one quote that will make any list I one day compile:

". . . never think that war, no matter how necessary, nor how justified, is not a crime. Ask the infantry and ask the dead." --Ernest Hemingway, Introduction to Treasury for the Free World, edited by Ben Raeburn (New York: Arco Publishing Company, 1946). [Thanks very much to Josh Silverstein of Timeless Hemingway for locating the source of the quote.]