Sailor Venus' Big Damn Heroes moment in Chapter 5, turning the tide of the fight against Jadeite's strike force.
The author's description of the Senshi's acomplishments shows just how awesome they are with what they have:
The Senshi are not trained professionals like Nanoha, Fate, and the Wolkenritter. In some ways that makes what they were able to accomplish more impressive. In modern terms, their actions are like a self-recruited, self-trained part time militia formed of people who had no combat experience beyond schoolyard scuffles before being recruited, no logistics, a shoestring budget, and the closest thing they have to medical support is someone who plans to apply to medical school at a point several years in the future. Said militia then ends up being the only line of defense against a considerably larger enemy force, with no way to detect an impending attack until it happens and no way to launch an counteroffensive, even if they knew where they were supposed to attack. And then they manage to successfully hold the line until real professionals learn of their situation – a year and a half later. There are undoubtedly a lot of things they could have done better. That doesn't in any way change the fact that they managed to accomplish that much is an astonishing achievement.
Miyuki in chapter 13. She's captured by someone she thought was her friend and turned out to be a dark general, and taken before the Big Bad of the series. What does she, an ordinary human in a story filled with city-busters, do? First, she comes an inch from killing Beryl with a pair of throwing knives, then uses her two swords she snuck in to take on a small army of youma, getting them to hit each other when she realizes she can't permanently hurt them. After all the youma have either killed each other or she's managed to put them down temporarily, she dodges all of Nephrite's shots, before charging Beryl again and almost disemboweling her. Beryl survives by mere chance. And she's an ordinary human. Shirou would be proud.
Takamachi Shirou, IE: Her dad in story, would also be proud.
And then she gets hold of an improvised weapon, which she later uses to break out of her cell during the final battle, and participates in the fight against Nephrite.
Nanoha calling out Queen Serenity's ghost on her many failings as a ruler and as a mother, even pointing out that the excuse "I was busy" doesn't really justify it for her- her own parents were either far busier or in the hospital, and they still managed to teach Nanoha about responsibility, manners, and more.
Fate's final showdown with Precia in chapter twelve. It's every bit as heart-wrenching and dramatic as the moment deserves, complete with a single devastating line.