In the dueling club. Lockhart's been talking non-stop with his usual self-importance and highly-inflated opinion of himself... and then he asks Snape to help him demonstrate a duel. Snape (who at this point has just been standing back and letting Lockhart make a fool of himself) casts Expelliarmus and not only disarms him, but smashes him into a wall — and what's even better, he actually impresses Harry "I hate them both. Malfoy and Snape" Potter. And, of course, gives Harry his signature spell.
Not to mention that this short-lived group, as incompetent as Lockhart was, might have served somewhat as the inspiration for Dumbledore's Army.
Harry in the Chamber of Secrets.
Tom Riddle sneered that all Dumbledore could give Harry was "a songbird and an old hat". However, it was those two things that guaranteed Harry's victory against the Basilisk.
All of Mrs. Weasley's sons were taller than her, but they cowered as her rage broke over them. It's a sign of things to come.
Harry telling off Tom Riddle about how his "filthy Mudblood mother" saved him, then killing off the basilisk and stabbing the diary.
When the basilisk emerges out of the statue's mouth, ready for battle.
Phoenix vs. Basilisk. And the crown goes not to the 60-foot snake with the killing stare, it goes to the "songbird".
We are introduced to the "songbird" first as Dumbledore's dying pet, then a feeble newborn. Months later, it reminds us that it is in fact, an immortal legend and one of the main reasons why Harry survives for so long. Giving two of his tail feathers rather than one protects Harry from Voldemort once he overcomes the shield on Harry, saving him from a Basilisk by intercepting its attack, blinding it and healing its venom, helping Dumbledore look after Harry...
Arthur Weasley and Lucius Malfoy's brawl in Flourish and Blotts. Words can't describe how much this troper wanted to see that get into the movie.
"Albus Dumbledore is the greatest sorcerer that ever lived!" Also doubles as a Crowning Moment of Heartwarming.
Mr. Weasley's car saving Harry, Ron and Fang from Aragog and his children, even though it abandoned them after getting beat up by the Whomping Willow.
Any time anyone takes Lockhart down a peg. Lockhart's annoying, idiotic and egotistical antics all throughout the story make the moments in which he is struck down very satisfying. To be more specific;
Hagrid's rant about Lockhart after he pays his hut a visit to offer his condescending "help". Even though this attitude does bite Hagrid in the ass later, it's still great to know that someone can see through Lockhart's BS.
Second is Snape cutting Lockhart down when he claims he "could whip up an Mandrake Restorative Draught in [his] sleep" -
Snape: Excuse me, but I believe I'm the Potions master in this school.
The third and best example would be pretty much every teacher in the school (minus Dumbledore) reduce Lockhart to a quivering wreck, when all of his Blatant Lies are thrown back at him and he can't dig himself out. The film adapts this sequence well, but in the book you can feel both the Professor's satisfaction and Lockhart's realization about just how badly he screwed himself over.
Ron going with Harry to 'find the spiders'. Ron's deathly terrified of spiders due to a hilarious, yet terrifying incident in his past. As someone with First-hand experience in conquering fears (in my case, heights), even going near what you fear takes a lot of courage. Tracking down what you fear the most on purpose just to save a friend? Kid has balls.
Snape is generally awesome, but especially when he sent Lockhart flying across the room in the dueling scene. But Snape topped himself in the "Holy fuck, that's awesome!" department just a few minutes later... just by SMIRKING at Lockhart. (The Smirk of Awesome happens at 3:16).
Right after that, Harry realises that the basilisk's venom will kill him. His last act is to destroy the diary, then tell Ginny how to escape and that everything will be alright, even when he believes he'll die. Of course, Fawkes ensures that he doesn't, but that is some impressive composure and dignity for a twelve-year old.
Dobby kicking Lucius's ass at the end.
Near the end of the movie, after Lucius Malfoy's plot was foiled:
Lucius: Well... let us hope that Mr. Potter will always be around to save the day. Harry: Don't worry. I will be.
Even cooler since Jason Isaacs said that this exchange was improvised.