The part where Scuttle and the other marine animals crashed the wedding and proceeded to beat up the disguised Ursula. Especially when Eric's dog Max bit her in the butt (probably as payback for her Kick the Dog moment), and she screamed the loudest of all. Pure hilarity.
The part when Sebastian drove that poor chef out of his skull. Call that part a Big Lipped Alligator Moment if you want. I call it hilarious.
"Le poisson, le poisson, hee hee hee, haw haw haw!!"
Seconded, that is the funniest part of the whole movie
And Carlotta's reaction is hilarious
Louie: *looks up and bumps his head; causing a butcher knife to nearly fall on him*
Carlotta: ''What are you doing?!
Louie: Well, I was just . . . uh, I was just. . .um. . .*sheepishly* I'm sorry, Madame *sheepish grin while putting out a little fire on his whisker*
Carlotta: Hmmph *grabs the tray and stomps out of the kitchen*
When Grimsby lifts the cover from his dish of stuffed crab... and it's Sebastian. Cue Ariel hastily gesturing Sebastian onto her own dish while Grimsby and Eric are distracted talking to one another. Grimsby unwittingly provides the punch line:
Grimsby: Wonderful! Now, let's eat, before this crab wanders off my plate.
[fork tinks against an empty plate with a little trail of stuffing marking Sebastian's escape]
"And never underestimate the importance of...BODY LANGUAGE!"
Ariel striking a sexy pose when Scuttle dresses her up in a boat sail and rope.
Bonus points for the sexy saxophone riff that plays.
Ariel drives like crazy. Bonus points for her expression, which shows she's thoroughly enjoying it. Extra bonus points for Eric's scared reaction, as well as his little slow popping up near the end, with an expression that said 'We're still alive?'
When Max sniffs Ariel out at the side of Eric's ship, she hides for a moment, then turns back and ends up face to face with Max. The expression on her face◊ is adorably funny.
Scuttle looks through the wrong end of the telescope at Ariel and Flounder at the beginning of the first film.
Then when he lowers it and sees Ariel close to his face, he says "Whoa, what a swim."
Also, Scuttle's attempt to play the "Snarfblatt" (actually a smoking pipe) that Ariel found.
Ariel brushing her hair with a "dinglehopper" with absolute glee in the middle of dinner, right in front of Eric and Grimsley. The latter two's expressions are just priceless, especially Eric's Fascinating Eyebrow.
Ariel taking Grimsley's smoking pipe and blowing smoke into his face. His expression with smoke all over his face is hilarious as is the fact that Eric and Carlotta both laugh upon seeing this.
Eric puts forth "Mildred" as a possible name for Ariel. She makes the most wonderful face to show her disapproval. It really points out how much you can communicate without using words.
Chef Louie chasing Sebastian on the Wedding boat.
Ending with Sebastian snapping the rope to let a mast swing down and knock out Louie's teeth.
The beginning of the movie, where Sebastian pulls a huge book of music from a tiny shell.
Earlier than that, after Triton makes his grand entrance, Sebastian enters to the same epic music—played on a kazoo.
Scuttle's attempt at serenading to get Ariel and Eric to kiss, which makes Ariel Face Palm.
During "Kiss The Girl" when he tries to sing and the flamingos stop him from continuing
When Sebastian attempts to speak to Triton about Ariel being in love, telling himself not to overreact.
Sebastian:(high-pitched crack) Yes...
From the third film, when Ariel and Flounder find Sebastian's underground music club.
The follow-up scene, with Sebastian sobbing his eyes out on the hem of Ariel's dress, is PRICELESS.
Not to mention when Sebastian blows his nose into Ariel's dress.
A reprisal of the chase scene in the second movie between Louis and Sebastian during the party, starting with a You Exclamation from Louis and a small "You" from Sebastian.
Melody mimicking Sebastian, with Sebastian himself topping it off with a blunt "Stop dat." The way he delivers that line is too rich.
The argument between Tip and Dash, the penguin and walrus respectively, from the second movie. When Dash says "She doesn't like birds who don't fly!", Tip gives a dramatic gasp and says he can't fly either. Then when Dash says he walks funny, Tip thinks for a few seconds...then screams the same response: he can't fly either!