Glinda: You heard me! (while gesturing arms to spell C-P-T-V-T) Captivity!
Another point for her would be:
Galinda: And he's been thinking, which really worries me.
Which is followed by
Fiyero: Uh, listen, I've been thinking...
Elphaba: Yes, I've heard.
Actually put her down for damn near everything said by G(a)linda because it was written for Kristin Chenoweth and she's always adorably hilarious. Such as the following:
Galinda: Doctor Dillamond, why do you keep talking to us about the past? You're supposed to be teaching us history. (Everyone, especially Elphaba, give her a long look.)
Fiyero literally (at least in some productions) comes swinging onto the scene, screaming "LET THE GREEN GIRL GO!" It's hilarious, considering how serious the scene is...as is the catfight between Glinda and Elphaba beforehand.
The following exchange between Elphie and Glinda:
Glinda: And what have you accomplished besides flying around on that mangy old thing!?
Elphaba: Well we can't all come and go by bubble! Who's idea was that? The Wizard's? Of course! Even if it weren't he'd still take credit for it.
Elphaba: Oh ho. Now wait just a clock tick! I know that it is difficult for that blissful little blonde brain of yours to comprehend that someone like him could choose someone like me! But it's real, it's happened, and you can wave that ridiculous little wand of yours all you want you can't change it! He doesn't belong to you, he doesn't love you! He loves me!
That whole scene, really. Later, Fiyero somehow manages to swing in on a vine in a cornfield. Even Glinda's confused by this.
"Something's wrong...I didn't get my way..."
"It's SHOES, Elphaba. Let GO, already." Considering how dark the scene is, and what it's regarding... absolutely HILARIOUS!
Galinda: Now, let's tell each other a secret that we've never told anyone before! Fiyero and I are getting married! Elphaba: He's asked you already? Galinda: (gleefully) Oh, he doesn't know yet!
From the song "What Is This Feeling?":
Both: There's been some confusion Over rooming here at Shiz: Elphaba: But of course, I'll care for Nessa. Galinda: But of course, I'll rise above it. Both: For I know that's how you'd want me to respond. (Spoken:) Yes There's been some confusion For you see, my roommate is— Galinda: Unusually and exceedingly peculiar and altogether quite impossible to describe— Elphaba: Blonde.
Also, almost the entirety of "Popular". "La la, laa-aa, laaaaa!" For certain the moment when Galinda tries to use her new wand to turn Elphaba's frock into a "pretty ball gown".
Galinda: (shakes wand) "Is this thing on?" (tosses wand aside) "Wear the frock, it's pretty!"
It's doubly hilarious when she tries to do it before the above quote.
Galinda: *ahem* (in a lilting, fairy-like voice) Ball gown. *nothing happens* *ahem* (In a decidedly LESS fairy-like voice) BALL GOWN!!!
This little gem:
Fiyero: You think I'm really stupid, don't you?
Elphaba: No, not really stupid.
From the book, when poor Boq confesses his feelings to Galinda, and Elphaba is snarking at him the whole way for sounding rehearsed. Galinda scolds her, while at the same point ripping on Boq with "Let me consider his stupid idea", while he's still sitting right there.
Nessa's hand gestures (indicating shortness) when mentioning Boq during "Dancing Through Life".
In addition to this, in Kristin Chenoweth's final performance as Glinda, she throws in a little surprise for everyone, including most of the backstage crew, in the form of a book called "How to be Popular"! She had Idina Menzel in stitches, and somehow managed to keep a straight face through it all! Can be seen here: http://www.youtube.com/watch?v=CUOiios1J6o