I can't narrow it down to one film, since they all have great dialogue, at least somewhere.
However, one piece of dialogue that I always liked was Kirk's exchange with Carol Marcus on Regula.
Kirk: "There's a man I haven't seen in 15 years who's trying to kill me. You show me a son who'd be happy to help him. My son. My life that could have been, and wasn't. What am I feeling? I'm old, and worn out."