Hermione getting her reputation. After breaking a Ravenclaw bully's wand, she claims that muggles are better fighters than wizards because they know true pain. She then describes what her parents do. They are Dentists. Word quickly spreads not to mess with Hermione Granger.
Harry, Ron and Hermione facing Quirrel and Voldemort in the Infirmary in 29.
After barely surviving a fight with Quirrel, Harry throws a bedpan at Volauvent's fleeing soul.
The massive Oh, Crap! moment the Death Eaters have after Harry rises up behind Voldemort, after surviving a second killing curse at only twelve years old, and casts a cutting charm at Voldemort. Some consider career changes on the spot.
Snape manipulating Malfoy and Rita Skeeter in chapter 19.
How McGonagall procures the Fake!Locket horcrux: she transfigures several beetles into elephants and has them suck up the potion that the locket was near and deposit it in a tank she summons. This works and neither she nor Dumbledore are hurt in the process.
Snape finally trapping Lucius into working for him. After Pansy ends up with the diary Snape asks Lucius if he knows just what family he messed with. Lucius doesn't think much of the Parkinsons but Snape reminds him that he's forgetting Pansy's material family... namely the Borgia, aka the deadliest assassins in the world, known for their poisons. And after pointing out that they might eye Draco up as a target Lucius practically begs Snape for protection
The moment Snape reveals Voldemort is a half blood Pansy's mother promises to give Snape the full might of the Borgia to destroy Voldemort in revenge for what he almost did to Pansy.