Well, The Motion Picure is my favourite Trek film but it's primarily a mood piece/visual experience so would be a poor pick for this topic. I'd say Wrath of Khan has the best dialogue of all the Trek films. All the character interaction is memorable and endearing.
Spock: If I may be so bold, it was a mistake for you to accept promotion. Commanding a starship is your first, best destiny; anything else is a waste of material.
Kirk: I would not presume to debate you.
Spock: That is wise. Were I to invoke logic, however, logic clearly dictates that the needs of the many outweigh the needs of the few.
Kirk: Or the one.
Spock: You are my superior officer. You are also my friend. I have been and always shall be yours.