A small epiphany: "Two good friends." "Two *best* friends."
Throughout the show there are many references to "goodness" especially from Galinda who seems especially fixated on "good" things. To her eventual detriment.
Meanwhile, Elphaba is the opposite, fully unconcerned with "goodness" so long as she does what she feels is right.
We see this play out literally during an early exchange: "Pink goes good with green." to which Elphaba corrects her: "Pink goes well with green." Emphasizing that "good" isn't always right.
However, in Oz when their friendship has deepened further, Elphaba finally admits to being "good friends" with what I now see is a very intentional use of "good". She's telling Galinda that she sees her and what's important to her, and that together, they are "good". Aka the thing most important to Galinda.
But then, Galinda corrects her. In the inverse of Elphaba's earlier correction from "good" to "well" to be grammatically correct, now Galinda corrects Elphaba from "good" to "best". This is her recognizing and agreeing that there are things more important than goodness, like their true friendship. It marks the first time Galinda strays from "goodness" and though it's just in language, I think it signals something much greater between the two of them, especially in context of their earlier convo.
Thought I'd share as it makes this already powerful interaction even more powerful for me.