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 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.
In chapter 39, Ron kills a Dementor, a feat that has never been done, with the Sword of Gryffindor, to save his and Hermione's souls. And then he taunts all of the other Dementors.
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...
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.