Perfuma nearly brings down the Horde singlehanded by simply being herself and driving Hordak crazy in "Flowers For Hordak".
Shadow Weaver nearly crushes Bright Moon and the rebellion in a single stroke in "The Crystal Castle". Executing a perfect plan and overpowering Glimmer, Broom and Madame Razz in the process. She-Ra had to cheat with time travel to save the day.
The same episode is also one for Queen Angella, with her castle drained of its protective power and her allies defeated Angella alone managed to hold off Shadow Weaver's assault long enough for She-Ra to save them.
In "The Stone in the Sword" Adora breaks the jewel in her sword protecting the Whispering Woods and can't transform anymore only to discover that Bo has been captured by the Horde. Adora proceeds to climb Sky Dancer mountain and pass several physically and mentally challenging tests to repair her sword, all for the sole purpose of saving her friend.
Glimmer holds off Shadow Weaver and a bunch of Horde troopers for nearly an entire episode despite her powers being extremely taxing on her.
She-Ra's transformation in general is awesome, but especially that first time.
The creators of the show wanted to establish both She-Ra and Adora as bona fide Action Girls, as opposed to the duality of her brother He-Man's MANLINESS versus Adam's (percieved) cowardice. So in Part 4 of the Five-Episode Pilot, while Adora's in Eternia meeting her parents, she gets captured by the Villain Team Up of Skeletor and Hordak. After a bit of backstabbing from Skeletor leaves her solely at his mercy, she decides to play the Distressed Damsel card by Fainting, "Just like a woman!" Skeletor has Beast-Man, largely considered his most imbecilic subordinate (and that's saying something!), carry Adora to the dungeon...where she promptly drops the act and locks him in the cell in her place.
Beast-Man: "Hey! You can't do this to me!"/
Adora: "Funny...I thought I just did it! 'Bye now!"/
Another example from later in the series was "The Stone In The Sword," where Adora has to repair her Sword of Protection by going through a series of tests, both physical and mental, without the powers of She-Ra. She succeeds in ways Adam never could have.
Shadow Weaver gets one in "Of Shadows and Skulls" when she shows what happens to people when she finally gets pushed too far. Summed up, the defeat of Hordak, She-Ra and Skeletor and gets away with it. Of course she also has several others (single handedly bringing Bright Moon to the brink of defeat, ensnaring all the leaders of the rebellion in a sleep spell and banishing She-Ra to an unknown dimension, wrapping the Whispering Woods in the mist of endless sleep, etc., even defeating He-Man) but that's probably her best episode.