The scene from "The Reckoning" in which Xena, encouraged by Ares, summons a sudden burst of strength, snapping the chains which hold her prisoner and kicking down the door of her cell.
Xena's ultimate solution to the problem, though much less action-packed, counts as well. The whole point of framing Xena for attacking and murdering men that she was actually trying to save turns out to have been a plot by Ares to force her to his side. She need only call his assistance and he will give her an army of anyone she asks for, living or dead. She agrees that it's a pretty good deal and says that she'll call for him only if she's found guilty, something he assures her will happen. Next morning, sure enough, she's found guilty. She turns to Ares, standing among the assembled crowd, and names her desired defenders— the innocent men he killed, who speak up on her behalf. Also a moment of heartwarming as the men are reunited with their families.
Gabrielle beating the living shit out of several mooks in a rage during "The Greater Good".
Callisto has a few good ones.
Her introduction during the teaser for 'Callisto'.
Callisto: I'm Xena. Warrior Princess.
Catching Xena's chakram in 'Callisto' was also pretty amazing.
Callisto: You want it? Come and get it.
Gabrielle blocking an arrow with her staff in 'Death Mask', before Xena even got a chance to catch it.
If there's anyone else among you who wants to give up their own child, speak up now because I will not have you fight next to me.
Hercules killing Zeus, and Zeus killing Hera in 'God Fearing Child'.
Xena killing Athena in Motherhood.
And their dialogue before hand. Particularily calm-cool-and-collected Athena losing it.
Xena: Goddes of wisdom, you got two choices- either you heal my friends, and I leave you in peace- or you die now.
Athena: I WILL NOT BE DICTATED TO BY A MORTAL!!
Xena killing Yodoshi in the finale.
Also Gabrielle's ladder trick, which puts out a massive fire consuming the Japanese town of Higuchi, saving thousands of lives.
Gabrielle pulling off Xena's super flip was also pretty awesome.
Also in the finale, Gabrielle catching the Chakram for the first time.
And Gabrielle retrieving Xena's ashes from the birds nest via awesome-warrior-hackeysack skills.
Despite the fans awe inspiring (and almost universal) hatred of the finale, the whole two part episode was practically made of these.
And how about Joxer in ''For Him The Bell Tolls"? Kicking off with his sudden Ali-shuffle before beating the crap out of several Mooks, his general awesomeness under the Bell's spell for most of the episode, then Xena's gentle advice to him as he sits despondent that the Bell merely brought out the genuine heroism inside him...followed by his sprightly bouncing to his feet and introducing us all to the ever-epic "Joxer The Mighty"...
"The Quest". Gabrielle looks like she's gonna hand over the Amazon Queen's Mask to Valeska, then pulls back and puts it on herself. Gabrielle - one, Valeska - OWNED.
Ephiny's Last Stand. Amarice might be obnoxious, arrogant, and disobedient, and Ephiny might not even like her—but she's one of her warriors, and Ephiny is not going to stand by and let her be killed because she she picked the wrong fight.
Xena. Callisto. Ladder fight ("Callisto"). Any questions?
Najara in her first episode ("Crusader") takes on Xena in a fair fight - and wins.
Callisto is Xena's nemesis motivated by a desire for revenge for the suffering Xena has caused in her own dark past. Before Xena reformed, she was responsible for the death of Callisto's family when she had her army torch Callisto's village, thus causing a chain of events that lead her to become evil and insane. At the end of her journey she was a demon and was fighting archangel Xena; Callisto says "I will never stop hating you, Xena. Do you hear me, never. You killed my family, my soul, my reason to live and love, and I will spend eternity seeking revenge" and Xena simply says "No," and gives her light to her turning into what she would had become if Xena wouldn't had killed her family, effectively trading her place in Heaven for Callisto's spot in Hell to make up for it.
There are some awesome heartwarming moments in that episode as well. Mostly surrounding Callisto.
That stirring rendition of "Sisters are doing it for themselves" with a very pregnant Lucy Lawless.