Astro's stunned reaction to the first time his rocket boots fire, including a hilarious expression and a wonderfully prompt (and pretty harsh) slap to his own face.
And then as shock becomes delight he tries to zip off heroically - only to briefly forget how to power the jets and, on reactivating them, slam himself backwards into a building and then straight up through its stone facade.
A soldier trying to use windshield wipers when Astro crashes into his ship. Astro's look of sheer annoyance is priceless.
Trash Can the robot dog's final attempt to "out" Toby as a robot, spelling out on the ground in huge letters "HE'S A ROBOT!" with an arrow pointing to Astro/Toby to Zane. Made funnier when Zane shrugs, "Almost makes me wish I could read!"
When the RRF 'rescue' Astro.
Zane: What just happened?
"I'm voting for the other guy!"
When Astro descovers his "accessories".
Astro: What? I got machine guns? In my butt?!
And the first use of his Arm Cannons: "Hah! Cool!" *blam!* [goes flying backwards]
When Astro saves one of the guy's after him, and then has to deal with the reinforcements.
Astro: What is up with you guys?
Guard: I love you man.
Any scene involving the Robot Revelutionary Front.