Pre-Crisis Kara's Dying Moment of Awesome, which singlehandedly set the Antimonitor's plans back an immeasurable amount, saved Superman's life, and ultimately, all of creation.
During the finale of New Krypton, Ursa has been visiting one capital after another and butchering world leaders. Arriving at the White House she kills Codename: Liberty and heads inside, only to be intercepted by Kara, who engages her in battle inside the Oval Office, and with an assist from Superboy and Krypto, brings her rampage to a halt.
Not Kara's only CMOA during that arc, either. Towards the end of it, Kara leads the charge into General Lane's headquarters, defeats Superwoman/Lucy in a couple of punches, and then takes down Lane himself.
Her first defeat of Superwoman, in her own book, is even better. After a long, gruelling fight, Kara pins Lucy, and tears off her S-Shield, declaring she has no right to wear it.
Calling out her mother on all of her bullshit and actually trying to save Reactron, even though god knows, he doesn't deserve it.
Earlier in the arc, during Last Stand of New Krypton, refusing to let Brainiac 5 sacrifice himself for her sake, and carrying him along under one arm, while she smashes through the original Brainiac's defenses with her free hand.
Kara going into Unstoppable Rage and laying a massive beatdown on Doomsday during Reign of Doomsday.
Kara: "I've been hoping for this...dreaming of the day when I'd get another crack at you. Not me and two dozen Kryptonians. Not me and my cousin. Just me. Supergirl! And Rao, it feels so good!"
In issue 23, Cyborg Superman sends robot replicas of everyone from Supergirl's memories to stop her from escaping. It is a sight to see her tear through the horde of robot knockoffs. She rips fake Superman in half, crushes fake Wonder Woman with asteroids, incinerates fake Siobhan with heat vision, and wrecks many more ripoffs before she's overwhelmed and captured by Cyborg Superman. She probably would've destroyed them all if it wasn't for her kryptonite poisoning acting up again.
In issue 24, when her mind was trapped in the I'noxia collective, her thirst for revenge against Cyborg Superman allowed her to will herself free and forms a humongous, incredibly powerful version of herself out of the collective's nanobots to get through Brainiac's forces and exact her revenge.