I have a couple nominations for this category. The first is found in the Woody Allen film, "The Front," in which he has been hailed before a congressional committe (in the 1950s) to investigate television scripts written by black-listed writers for whom Woody is a "front." At the close of the film, after trying to adroitly avoid giving direct answers to the committee's questions, Allen starts to walk out of the meeting room, saying "fellas, I don't recognize the right of this committee to ask me these kinds of questions. . . (pause) And furthermore, you can all go f*** yourselves." (I hate this kind of *** self-censorship, but out of respect for the blogsite, etc.)
Another favorite line is in "Doctor Zhivago" where, towards the end of the film, Zhivago's brother - played by Alec Guinness - is trying to convince a young woman that she is Dr. Zhivago's long-lost daughter. The girl is skeptical. Zhivago's brother tells her what a great man and great poet his brother was, then asks: "why won't you believe it? Don't you want to believe it?" "Not if it isn't true," the girl replies.
One of my favorite scenes in a movie is found in one of the greatest anti-war films ever: "The Americanization of Emily," and takes place in the garden of the mother of the Julie Andrews character. The mother - who has lost her husband in the war - is prattling on about war generally, when the character played by James Garner - a Navy officer - takes off on a rather lengthy (and pointed) attack on the war system. I shall not try to repeat it from memory, but urge all who have not seen this film to rent/buy a copy!