The moment when Alex realizes he's locked in the back of a car being driven way too fast by a man he thinks is completely insane. (he doesn't know the half of it).
Centauri: (cheerful singsong) That's why I'm not gonna tellll youuuu... Alex: Oh God.
Louis' disgusted reaction to Alex and Maggie snogging: "Diarrhea!"
"Back to sleep Louis or I'm telling mom about your Playboys!" ...and (repeat).
Between Grig, Beta and Centauri, quite a few, the crown jewel being:
Alex: It'll be a slaughter! Grig: That's the spirit! Alex: No, my slaughter! One Gunstar against the armada?! Grig: Yes, one Gunstar against the armada. I've always wanted to fight a desperate battle against incredible odds! (Alex just sits there, looking absolutely incredulous)
Also, this exchange:
Grig: I'll have it all figured out by the time we reach the Frontier... (Alarm beeping as Grig's face drops) Alex: What's that? Grig: The Frontier.
Not to mention when Alex finds out he IS the Last Starfighter.
For instance, there is Beta repairing his detached head and dealing with Alex's brother ("You're having a terrible nightmare. Go back to sleep.") just before he sees (to his pained consternation) an assassin landing. Then there's Beta finally giving up and telling Maggie that he's an android, a claim proven moments later when he's shot, revealing his circuitry and squirty innards. His only reaction is a casual "See?" before he gives chase to the shooter.
His reaction when Alex snaps he doesn't want to be at war. "Ah, I see. Save the whales but not the universe?"
What did Beta do to Maggie to cause her to slap Alex next time she saw him? She stuck her tongue in his ear, and he freaked out.
Beta eavesdropping on Jack Blake and using him as a role model for how to talk to Maggie. Comparing her to Venus seemed to turn her on. Saying the other girls didn't mean anything to him, not so much.
In a Funny Background Event, as Alex is annihlating the Ko-Dan fleet in the video game, going for the record, and the whole trailer park comes out to watch. The Command Ship looms into view on the screen, and everyone is in awe that he's reached the command ship and is preparing to attack. One little old lady in the crowd gasps "Ooh, Command Ship! What's a command ship?" Even funnier is an old man describing it as "Air Force One for the aliens."
The implication that life in the trailer park is so boring that watching Alex playing the game is literally the most exciting thing any of these people have going on.