This one's kind of tough. I feel like word selection is actually the weakest aspect of my writing— I'm better in general at the design portion of it, coming up with the ideas behind it, structuring it, shaping it. And as always, it's a hard thing to quantify. I have a whole lesson in my English classes about how the "picking of good words" part of writing is basically impossible for me to teach. How do you measure it? What rules do you apply? Especially when so much of it is contextual, and moreover a matter of taste. But every now and then, I do think I hit on something that sounds genuinely good.
I find myself struggling to find the perfect one, but let's not make the perfect the enemy of the good. I'm going to think of things that made me comment "That's a good line," out loud to other people, at enormous risk of sounding like an egomaniac, when I heard them spoken. And a few in recent memory come from a particular scene in Mrs. Hawking part V: Mrs. Frost— specifically, scene 2.5, when Mrs. Frost and Mrs. Hawking finally face off in person after gunning for each other from a distance the entire play.

Photo by Daniel Fox
By this point, the audience has been waiting for the two of them to interact directly, so there's a lot riding on the scene, both in terms of the story and of the audience's engagement. So I wanted their interplay to be crackling. I got a lot of good ones in there, mostly from Mrs. Frost. She knows our hero well enough to see and speaks her weaknesses and fears, in order to cut to the heart of where she is most vulnerable.
She gives name to some of the discomfort Mrs. Hawking has dealing with Nathaniel. "It’s remarkable, you know— in the right light, he could be your husband. In the right light, he could be your son."
She knows the worst, weakest, and most pitiful of Mrs. Hawking, and reminds her of it. "You are still as powerless as when last I saw you, a girl trembling in nameless dread of your wedding night."
But I'd have to pick this one: "Fate falls hard upon a hero’s shoulders. Small wonder you’re always raging— at your father, your husband, and me. But you ought to be grateful. We made you what you are." It voices the great tragedy of Mrs. Hawking's existence, that most everything that she holds dear as part of her identity is the direct result of the worst things that ever happened to her.
But the thing that's really notable about these lines is that they sound good, in that unquantifiable way that poetry can. The rhythm is good. The music of the words. They stick in your mind. I'm not sure they're the most poetic I've ever written. But when I heard Arielle say them, I informed everyone in the room, "That's a good line."