Any instance where Guybrush uses the ventriloquism book to throw his voice in Part II, but of particular note is when he uses it on the monkey pirate captain and does a Captain Kirk impression:
Guybrush: (as LeChimp) I mean, take a look around, at me, at the rest of the crew... We're all monkeys. Mr. Fossey: You mean in the Darwinian sense, sir? '''Guybrush: No, I mean in the quite literal sense.
The reveal that Mr. Fossey's captain is not LeChuck but LeChimp, a mute, dopey looking gorilla modelled after Captain Kirk.
Guybrush: But If the captain is a gorilla, then that must mean Mr. Fossey is... Mr. Fossey: Aye aye, sir! Free bananas for the whole crew! Guybrush: (deadpan) ...An utter loon.
In the sequence where Guybrush fakes his death a second time to get buried in the Goodsoup crypt, hotel manager Griswold Goodsoup offers this gem:
Griswold: Oh, dear! He's had a sudden and completely unexpected relapse of death!
"You fool! You've given cheese to a lactose-intolerant volcano god!"
On that note, Lemonhead in general. "Shut up. Or I'll eat you."
Guybrush: Did I say "cannibal?" I, I meant to say... err, "cannon... ball." I want to be a cannon ball. BOOOOOMM!
Lemonhead: ...You're not quite stable, are you?
Similarly, after the "human sacrifice" to the volcano god (which is actually a dummy made out of fruit), complete with dramatic ritual-saying and rope-cutting, the cannibals stand around the volcano's edge and toast marshmallows.
The "terrifying" shape of Skull Island.
Lost Welshman: Well, ya see... ya gotta squint and sorta turn your head and... Ooooooooh! It's just SO scary!
Guybrush: If you squint and turn your head it looks like a bunny.
Asking LeChuck what the real secret of Monkey Island is.
Stan's triumphant return. Especially if you've played the preceding games.