Quite possibly everything said by Fat Amy and Lilly.
To put it into perspective, while driving to finals Lily sits behind the driver, Donald, who is beatboxing. Afterwards, she quietly joins in, to which he comments that she's pretty good. And then she goes out and says "I set fires to feel joy." In the sweetest, quietest voice possible.
And Donald's response:
And yet another Lilly moment, while the Bellas are confessing their issues and problems to each other, her chosen fun fact was that she'd eaten her twin in the womb.
Lilly's introduction at the audition. If you can read lips, or turn the volume up she says she has gills like a fish. Not only does she repeat that fact when they ask her to speak up, but she actually does a fish like twitch with her mouth.
Her disturbing non sequiturs make her firing an imaginary shotgun at the Treble Makers even more funny, because she could actually be serious.
The moment when Lilly finally speaks above a whisper.
Fat Amy: Excuse me, bitch, you don't need to shout.
A lot of the banter between John and Gail.
Fat Amy's horrified screaming when Aubrey finally loses it and vomits all over the place.
The Bellas practicing their choreography, with some hilarious vocal exercises and dance moves.
As part of a plan to rescue the kidnapped Bellas, Beca has to slip in with the rest of the group and initiate a distraction. When she starts speaking, Fergus immediately notices that she wasn't there before - her cover story is "I was here this whole time; I'm just very small".