I kept waiting and waiting for the mother of all grand-slam insults, and the only ones that popped through were two old-timers: (a) Joe Pesci‘s elephant dicks line from Raging Bull (starting at 5:03) and (b) Jack Nicholson ‘s withering analysis of female character in As Good As It Gets (starting at 4:23).
The best insults are (a) those that are delivered so deftly that the victim doesn’t realize he/she has been zinged until three or four seconds have elapsed, and (b) those that are coaxed out by the victim, and therefore spoken by the author with some reluctance.
My favorite is from Paths of Glory (1957) when George MacReady‘s General Mireau delivers some pompous line about French patriotism, and Kirk Douglas‘s Colonel Dax mutters that Samuel Johnson felt differently about it. MacReady asks him to explain and Douglas says it was nothing. MacReady angrily insists and Douglas says Johnson’s line about patriotism being “the last refuge of a scoundrel.”