Awesome / Harry Potter and the Nightmares of Futures Past
Arthur Weasley standing up to Dumbledore and telling him, no, Harry will not be taken back to the Dursleys, not under any circumstances.
In chapter 28, the twins save Harry from being hit by a Bludger in the nick of time: Fred sends the second Bludger to George, who manages to hit the Bludger with the other Bludger. This allows Harry to catch the Snitch and give the victory (and the Quidditch Cup) to Gryffindor.
Chapter 31: Harry massacring an horde of Dementors with summoning charms. OK, he had the Dementor-killing wards the Goblins had installed at the Burrow to do the killing part of the job, but still...
In chapter 33, Neville Longbottom standing up to his grandmother by sending her a Howler. Everybody within hearing range is shocked that anybody could send a Howler to Augusta Longbottom. And then one of the ladies realizes that the Howler is a result of her own son's gossiping.
Also, in the same chapter, Harry silencing Augusta's own Howler (the direct cause of Neville's Howler) and Neville's explaining to McGonagall his grandmother had sent it because Neville had "offended her bigoted, pure-blood sensibilities". And the group giving the gossiper the deserved punishment: even Hermione joined in, if only by teaching Harry a curse to make him look like an idiot.
And about that punishment... Harry tried to assure McGonagall everything was under control, nobody would do anything and smile reassuringly, but the teacher was terrified.
Poor woman was probably having flashbacks to the times when James Potter donned that very same smile.
Even better by applying some Fridge Brilliance. Remember how the Sword of Gryffindor was able to absorb the destructive properties of Basilisk venom? Well, if it works the same way, now the Sword of Gryffindor can absorb souls.
Slughorn showing everyone the cunning of a real Slytherin, apparently listening his House's false complaints against Harry before using them to justify his actions to prevent them from bullying.
At the end of the year, when Pansy Parkinson asks him why he is "sucking up to the Gryffindors", he shows her how much money he earns every year and tells her he got three times that a few weeks before because he was polite to Harry.
Slughorn: Now, which of us is the better Slytherin?
Harry manages to help capture Barty Crouch Jr by laying a trap on him: he puts one of Fred and George's fake wands in his pocket and waits for Barty to bite. Which he does. Spectacularly.
Thanks to Harry's influence, Percy becomes less anal about the Ministry, and is willing to attack his boss (whom he idolized in the original books, even though he never got his name right) when he starts acting weird. This helps him to land a much better job as Madam Bones' (the head of the Department of Magical Law Enforcement) assistant.
Thanks to Harry's idea, the twins get much good attention from Madam Bones, who promises to give them free publicity in exchange of a couple of their fake wands.