For me, it was when the invitations for Harry to attend Hogwarts came flooding in. Very original and funny.
When Dumbledore takes Harry to the cave in the sixth book, they cross the water, Dumbledore drinks the water, and they have the “zombie” horde rise up.
It was great. I think the sixth book is the best one by far.
Dumbledore v. Voldemort at the Ministry, which is also the most disappointing scene in the movies. Dumbledore calmly chatting with Tom while slinging spells was so much cooler than the fireball battle they turned it into in the movies.
Molly Weasley vs. Bellatrix Lestrange. Goosebumps plus I actually pumped the air when I read it.
The best scene in the books is without doubt when Harry returns from the graveyard via portkey with the body of Cedric Diggery. For the first time in the books, the fantasy of Hogwarts becomes the serious stuff of real life, death and all. It was a well-written scene, and was done well in the movie, too.
Ron and Harry on Christmas morning at Hogwarts.
I was going to mention this, as well.
The twins, followed close upon by Peeves, getting the better of Delores Umbridge.
When Narcissa Malfoy betrays Voldemort, in order to protect her son.
I liked the visit to St Mungo’s Hospital for Magical Maladies and Injuries. Arthur Weasley is so describing when he’s describing to Molly his foray into"alternative" medicine (stitches). I also thought the visit with Gilderoy Lockheart was fun. I’ve always liked him.
Becomes a downer when they meet the Longbottoms though.
I also like the opening scene in Half Blood Prince. Just when you have convinced yourself that Snape might actually be one of the good guys, he persuasively argues that he is a triple agent, so the reader is again left in doubt about him.
My favorite scenes involve George and Fred’s practical jokes.
I like the descriptions of the Quidditch games, and also of flying in general.
Having a flying broom would almost be like a real-life jetpack without all the noise and other unpleasant stuff.
When Hermione takes responsibility in the mountain troll debacle. It’s when she aligns herself with Harry and Ron.
One of my favorite scenes is when Hagrid takes Harry to Diagon Alley for the first time, and he gets his first taste of the wizarding world.
Judging by number of re-reads, these are my favorite scenes, too. Closely followed by every happy Christmas morning. And the first look at The Burrow.
I opened the thread just to bring up that scene. Good on ya for beating me to it. 
A little more detail about why I agree with this: I approached the Harry Potter books with the thought that I was reading kids’ books, and that they were likely to be silly and juvenile. The Hogwarts invitations were my first clue that these books were oiginal, and that I was probably going to like them.
The scene that leaps to mind at the moment is Harry’s career counseling session with Professors McGonnagle and (boo, hiss) Umbridge, which features: “Potter, I will assist you to become an Auror if it is the last thing I do! If I have to coach you nightly, I will make sure you achieve the required results!”
I have to confess to having two:
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when Hermione punches Draco, and
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Emma Watson when we see her all dolled up for the first time, at the Triwizard party. Then again, I think that’s where I discovered my crush on her. Yeah, that’s the movies, but damn.
Honorable mention to Harry against Voldemort in “Goblet of Fire” - even though I’ve always wondered why Harry chose to compete at all. What would have happened if he’d just skipped all the contests?