When Neptune rhetorically asks Mindy why his crown is important, Mindy guesses it's to cover up his bald spot. He insists that he's not bald, just...thinning, and that the crown is a symbol of authority.
Neptune: Someday, you will wear this crown.
Mindy: *horrified* I'm going to be bald!?
Three words. The Waiter Scene.
Spongebob: (is drunk on icecream) "Alright folks, this one goes out to my to bestest friends in the whole world! Patrick and this big peanut guy! It's a little diddy called, WAAIIIIITER!!!" (falls over)
The morning after. Oh dear GOD, the morning after. SpongeBob wakes up on the floor of Goofy Goober's Ice Cream Party Boat, which is now a complete wreck. The guy clearly has a hangover, his slurred speech and drunken behaviour are a hilarity to watch. Especially when he angrily remembers about Mr Krabs not promoting him to Manager, and when he notices Patrick unconscious under a table - Tom Kenny's delivery of this line is PERFECT:
SpongeBob later shows up at the Krusty Krab 2, where King Neptune has just arrived accusing Mr Krabs of stealing his crown. Still hungover (and mistaking the KING for a woman), he tells off Mr Krabs for thinking that he's still a kid:
SpongeBob: I'VE got somethin' ta say about Mr...(holds back vomit) Kraaabs!
Mr Krabs: SpongeBob, me boy! You've come just in time! Please, tell King Neptune all about me.
SpongeBob: I have worked for Mr Krabs for ma-many years, and always thought he was a great boss!
Mr Krabs:(to King Neptune) You see? A great boss!
SpongeBob: I NOW realise that he's a GREAT BIG JERK!!!(Mr Krabs turns to him, shell-shocked) I DESERVED THAT MANAGER'S JOB! Butyoudidn'tgiveittome! Because you said I'm a...KID! Well I am one hundred per-cent MAAAN! And this MAAAN has got somethin' ta saaay to yoooouuuu! (blows gigantic raspberry, then smiles smugly) THERE! I think I made my point!
Mr Krabs' face in that scene. Priceless.
Not to mention the music used during the whole time SpongeBob is hungover: "Too Tired", which is pretty much the theme song of any hangover ever.
The Mariachi fish.
The Cyclops in general. There's just something really absurd about a guy who runs a seaside gift shop walking around in an old-fashioned diver's suit above water.
"All bubble blowing babies will be beaten senseless by all able-bodied patrons in the bar"
The faces Spongebob and Patrick make during certain points in the film, like when they discover the Patty Wagon at the tough guy bar or when the monster in the trench swallows the "ice cream monster".
"Did you see my butt?"
Patrick: Did you see my underwear?
Mindy: No, Patrick.
Patrick: (holding shorts) Did you want to?
"So...you think I'm...stalling?"
The scream Neptune makes when he finds out that his crown is in Shell City:
Neptune: MY CROWN IS IN THE FORBIDDEN SHELL CITY!?
"WHERE AM I, IN CRAZYTOWN?!" An interview revealed that this was Neptune's VA Jeffrey Tambor's favorite line in the movie.