Most of the major characters get one at some point, but one standout is Dairine in High Wizardy. She's going head to head with the Lone Power and trapping it within the darkness of her soul, which she does by stopping the expansion of the universe, causing all of the photons trapped in the expanding space-time to suddenly get wherever it was they were going much faster
Nita's mom in Wizard's Dilemma:
"This is my universe, and I say what goes here, and she does not have my permission.... My daughter and I are fighting the same battle. Maybe I do it in more ordinary ways. But we're on the same side. And you, if I recognize you correctly, are no friend of mine. Get off my turf!"
Kit has a Big Damn Heroes moment in this same book, where he turns up right when Nita has pretty much given up her wizardry, bringing along the old version of Neets's name, before she had made her Deal with the Devil, fixes it into the spell with the glede or "dragon's eye" and blows all the cancer right out of Nita's mom's system. Unfortunately this isn't enough to save Betty . . .
Nita in A Wizard Of Marsturning back a flood and then kicking the ass of the person who tried to kill her
More specifically, laying out and fueling the Gibraltar Passthrough, a transoceanic passthrough designed and used by the famous hydromage Angelina Pellegrino. She then proceeds to control two huge snakelike structures of water and threaten to wipe her rival's city off the face of Mars "like dog poop down the driveway"! Then she engages in what's supposed to be mortal combat with Aurilelde, tells off Aurilelde and "explains" to her the real meaning of love, and then yanks the kernel out of 'Lelde after Aurilelde had become the "war goddess" of Mars, and proceeds to calm down Mars, tell off the "Martians," and decide to save Aurilelde's life instead of following through with the prophecy.
Nita's Big Damn Heroes moment in A Wizard Alone where she pops into Darryl's universe with the kernel (which she's fixed so that nobody can mention Darryl being an abdal,) explodes all the glass windows with her energy weapon and manipulates the Lone Power into letting Kit and Darryl go.
Dairine organizing and leading a team of alien wizards to fix the Earth's Sun without any guidance or input from Senior wizards in Wizard's Holiday, closely followed by Roshaun's Crowning Moment of Awesome, stepping out of the protective wizardry and into the center of the Sun itself to tweak it on the molecular level, risking his own life for a star that wasn't his own. Good thing he's a master of star-based wizardry.
In the Feline Wizards book Book of Night with Moon, the main characters become incarnations of the Powers That Be, with Ith enacting the role of the Bright Serpent, to fight the Old Serpent.
And then the sharks came.
Ed: "LONE POWER, I ACCEPT YOUR GIFT!"
So rage, proud Power,
fail again,and see my bloodteach death to die!
Nita and Kit reading out of the Book of Night with Moon AND making all the statues in NYC come to life and Fred's sacrifice.
In DW, it's the battle of the krakens and, y'know, everything during the reenactment in the underwater canyon.