Batgirl's origins. She took to making up a Batman costume for herself and to win made it just the same, including working weapons and gadgets. Well on the way she encounters the villain Killer Moth, fights him, and wins. So thrilled by the experience Barbara goes into heroing full time, and she's so effective Batman himself basically gives carte blanch for her to do what she wants.
In World's Finest #169 Barbara meets Supergirl for first time when they're thrown into another dimension by Mr. Mxyzptlk and Bat-Mite. They not only manage break out on their own but also return in time to save Superman and Batman from a death trap.
Barbara Gordon as Oracle in general. For perspective, following her crippling in The Killing Joke, Barbara Gordon had been all but written out of the Bat-books and the DCU in general, and most people remembered her as Yvonne Craig in the Batman TV show from the 60s. After this, Oracle's become big enough to get several comic series including Birds of Prey, her own TV show of the same name, repeat Justice League memberships, and inspired the resurrection of a cheesy villain named the Calculator just to be her evil counterpart. She's generally considered one of the sexiest women in the DCU, and it's primarily for her personality and mind.
And of course, effortlessly brushing off Batman's warnings to shut down the Squad, sensibly pointing out that if he does go public with the info he gathered, so will she.
Cassandra's backstory is a testement to her incredible willpower and conviction. She was raised to kill, literally knowing nobody but assassins (and a single psychologist) her entire life. But as soon as she saw death for the first time, she didn't hesitate to leave everything she knew behind, with no idea as to where she was going, solely so that she would never have to kill again.
In her first outing as Batgirl, Cassandra took down Two-Face and his gang single-handedly, then left him catatonic by taking every single coin in his hideout.
Cassandra is left without her powers due to a telepathic accident, leaving her with no defensive capabilities. But when she spots Lady Shiva, the most feared killer on the planet, she doesn't hesitate to investigate her, knowing full well that if she's caught she doesn't stand a chance. When Shiva successfully lures her out and subjects her to a Curb-Stomp Battle, paralyzing her from the neck down, Cassandra stares down the woman considered by many to be the embodiment of death itself... and spits in her face. This brazen act of defiance, along with the fact that Cassandra is her own daughter, and therefore has incredible potential as a fighter, convinces Shiva to only gravely wound, instead of kill her.
Shortly afterwards, Cassandra wakes up in an ambulance covered in bruises, with several broken ribs and a broken arm. However, she recognizes that she has Shiva's necklace clutched in her hand. She then proceeds to leap out of the moving ambulance, track down Lady Shiva, and when Shiva moves to take the offered necklace, she punches her out with her broken arm.