A good portion of the hunt scene at the beginning. They manage to strike a balance between awesome and funny, but what sells it is:
Grug: Release the baby!
(cue Sandy charging after the horned bird like an attack dog)
"Eep! Avenge me!"
The whole thing is basically a looney tunes cartoon. Which makes it all the more funny if you imagine those cute little rodents as having dapper English accents.
The first encounter with the big-headed tiger. Grug throws a rock at it. It catches it and tosses it aside with a "Really?" expression on its face.
Really, every expression that tiger ever displays. After joining forces with Grug, who declares that he has an idea, Chunky very clearly looks worried to be near him. Them while helping Grug, he gets a clear "WTF" expression on his face at the tar pits. But the best one has to be when they successfully achieve flight, and Chunky's face slowly morphs from fear to an elated expression that screams holy shit this is working!
Eep getting shoes.
"I love them! Where are my feet?"
Grug's occasional lapses into Angrish.
Thunk with his pet crocopup.
Thunk: I'm gonna call you Douglas! Can you do tricks? Roll over!
(Douglas rolls over off the edge of a cliff and plummets to the bottom)
Thunk: ...good boy, Douglas!
On the road, Gran gets hungry and tries to eat Belt...who pulls a knife on her.
Guy's response makes it even better: "He will cut you!"
At the end of the movie, when everyone is posing for a family "snap-shot".
Ugga: If your father were here right now, he'd say—
Grug: (offscreen) GET OUT OF THE WAY!!!
Thunk: Wow, you sound just like him.
Both puppet shows.
"Here, hold this."
Grug's reason for trying to come up with ideas.
Grug: And...I thought it would kill your mother.
The prehistoric equivalent of "Heads or tails".
"Try hiding from it in the tall dry grass!"
During one of Guy's quirkier moments, Eep asks him how long he's been on his own. Cue Belt spreading his insanely long arms.
When Chunky appears behind Grug in the cave and his breath blows out the torch... and they both start it up again by frantically blowing on its embers, because Chunky's even more afraid of the dark than the people are.
Gran: "I was in love once. He was a hunter, I was a gatherer. It was quite a scandal."
Guy, on the coming "end of the world": "I don't mean to sound alarming here, but ARGHHHHHHH! AAAAARHHHHH! AAAAAAHHH!"
Grug repeatedly finding himself back at the place where he fell into the maze.