Fritz is probably the most awesome character in the movie. He takes on an army of polyps using starfish and a mermaid skull as weapons, and saves Marina multiple times.
The Disney film
The very first one in the whole movie and Ariel's Establishing Character Moment would probably be the shark scene in the beginning. Seeing Ariel outwitting a shark like that is pretty stinkin' badass and pretty much solidifies her as the first Disney Princess to be an Action Girl.
Eric does a LOT of stuff that no reasonable person would do, and comes off as a major Badass because of it.
The final battle, especially Eric who shows off how totally badass he is. His dispatchment of UrsulaDEFINITELY qualifies.
Hell, Eric has a ton of them over the course of the first movie, he's basically a prototype version of Aladdin.
Eric is insanely brave enough to destroy a freaking gargantuan human/cephalopod monster by stabbing it in its chest.You should never mess with him or his girl.
Scuttle, after being little more than the comic relief for the entire movie, is told: "find a way to stop that wedding!". Let's just say — it does not end well for Ursula in the slightest as Scuttle recruits the entirety of the ocean to rescue Ariel.
In that same scene, Eric's dog Max gets a Moment of Awesome (and some well deserved revenge!) by saving Scuttle from being strangled by Vanessa/Ursula by biting her in the ass!
Ariel rescuing her Prince from drowning was awesome! Granted he did save his damsel in the end, but you got to admit, Ariel is like the first Disney Princess to ever RESCUE her love interest!
Even more impressive, Ariel saves Eric's life again during the climax. Ursula points the trident at Eric and tells Ariel to "say goodbye to [her] sweetheart!" Quickly, Ariel grabs Ursula, changing the trident's aim so that the blast misses Eric for Flotsam and Jetsam.
Ursula is quite possibly the only villain to put together a plan and have it work completely. She suckered Ariel, traded her for Triton and took over the entire ocean.
Ursula's transformation from the beautiful Vanessa to the monstrous sea-witch. Seriously, only Ursula can mix Squick with pure awesome.
Ariel being naked from the waist down after her first transformation is pretty awesome.
Any time Triton uses his trident usually becomes a CMOA.
The level of attention the animators gave to Ariel's hair. Even 25 years later, it still holds up well.
Ariel herself is awesome as she is the first Disney Princess that actually did something and showed determination to make her dream come true.
Driven home by Ursula's little remark after the "Kiss The Girl" song. Maybe "little tramp" isn't quite the appropriate term, but can we all agree Ariel's having Eric ready to give her true love's kiss after two whole days was awesome?
As scary as it was,the way Ursula transforms into a gigantic version of herself is pretty damn awesome by itself. Her Family-Unfriendly Death too.
The moments when you realise that there are reasons why no-one in the entirety of the ocean fucks with Triton's daughters... Yipes.
The "In Harmony" number from "The Evil Manta." Ariel defeats the Manta's plans and drives him away... with one song.
Especially awesome when you remember that singing is Ariel's most noteworthy talent.
Earlier, Flounder gets one by tricking the Manta into thinking his brainwashing is working, then dropping the act the moment Ariel catches up to him.
In the episode "Thingamajigger", the Evil Manta gets his hands on the eponymous item (actually a boot someone dropped into the sea) so he can threaten King Triton with it. (At the moment the boot was found, a stray cannonball from ships fighting on the surface fell and destroyed the rocks the boot happened to be pointing at - so they assumed the boot did it!) Ariel, however, manages to scare him away by pointing two "Thingamajiggers" at him like a pair of guns. BootsAkimbo!
What's even better about it is that the "Thingamajiggers" Ariel pointed at Manta belonged to her future husband Eric.
In "Whale of a tale", Ariel adopts a baby killer whale, Spot, who got lost by his family. And he is too innocent, scared and childish to go back to the wild ocean. However, when three sharks attack Ariel, who has been like a mother to him, Spot gets mad. And the way he beats the crap out these terrifying sharks, is just... beautiful.
The music in this episode is so good it makes all the others look lame.
Whereas Eric saves the day, it's quite different in the stage/Broadway musical. Without her magic shell that she was given by her father, Poseidon, Ursula will die without it. In the climax, Ariel smashes the shell, destroying Ursula AND saving the day!