Tip and Dash saving Melody from nearly drowning to death. What makes it even more awesome is that they took down the Giant!Undertow, even after seeing him! These two may have started as cowards, but they ended up being true heroes.
Morgana raising her ice throne.
It may not be as a big or showy as the others, but Flounder standing up to Morgana's deserves mention. Think about it for a second: He's a tiny fish with no magical power and she's a powerful, octopus like witch creature who could kill Flounder if she saw fit. But, that doesn't stop him from (almost, if Undertow wouldn't have interrupted) giving her a piece of his mind.
In the climax, Melody taking on Morgana. A twelve-year-old girl with no magical powers, taking on a powerful, sorceress who has the ability to overpower and kill her. Melody didn't defeat the sea hag, but she this girl has more guts than someone twice her age.
Heck, the way she jumps and clambers all the way up a quickly rising ice throne is pretty cool, even before she gets the trident back.
Sebastian of all characters scaring away Undertow when he's still stuck in his piranha form.
Say whatever you want about the sequel,the way the merfolk guards rise from the sea and lock swords with each another to signal Triton's entrance is quite epic.
Melody manages to steal back the trident from Morgana and gives it to Triton so he can finish the job.
Tip and Dash taking on Undertow and saving Melody from drowning is also awesome.
Ariel saves Eric for the third time from Morgana's sting rays. Bonus points for Sebastian helping.
Ariel asks Scuttle to go get help. He returns with what can only be described as an army.