One of my favorites is from Sisko, but I guess this one is more of a soliloquy than a dialogue,

The trouble is Earth! On Earth, there is no poverty, no crime, no war. You look out the window of Starfleet Headquarters and you see paradise. Well it’s easy to be a saint in paradise, but the Maquis do not live in paradise! Out there, in the Demilitarized Zone, all the problems haven’t been solved yet! Out there, there are no saints! Just people! Angry, scared, determined people, who are going to do whatever it takes to survive, whether it meets with Federation approval or not!