I'm sure that, for people who come to articles of this sort, there was a moment when you became not just someone enjoying a book, but A Harry Potter Fan. Maybe it was when Hermione joined the Trio because of a twelve-foot mountain troll. Perhaps it had something to do with Quidditch. Maybe you read the books out of curiosity and didn't really get into them until you met Professor Lupin. I became a fan the moment Hagrid burst into tears in Chapter 1. Everyone comes to fandom a different way, but there are many points in the Harry Potter series that are great hooks. Let's review my candidates for the 7 best.
1. "It was Quirrell."

I didn't realize when my aunt sent me Harry Potter and the Philosopher's Stone that I needed to solve a mystery. It was just a children's book to keep me busy while I healed from an ankle injury. But it reminded me of a book my family had enjoyed when I was young. The Eleventh Hour by Graeme Base was a picture book where a magnificent feast was devoured by unknown intruders at a birthday party. In order to solve the mystery, you had to pay attention to playing cards on one page and sheet music on another Dialogue pointed to answers and we put together our theories carefully before breaking the seal at the end of the book and reading the answer.
When I got to "The Man With Two Faces," I was ready for the showdown with Professor Snape and a Dumbledore intervention that just had to happen. It turning out to be someone else entirely and the way that the final chapter unfolded proved that J.K. Rowling was wonderful at misdirection as well as making us recall things that we really should have taken more seriously in hindsight.
2. "Voldemort put a bit of himself in me?"

After his second encounter with Lord Voldemort in as many years, Harry has another chance to talk to discuss weighty matters with Professor Dumbledore. He feels something nagging at him and finally is open about his concerns. Foremost is the fact that Tom Riddle compared himself to Harry and mentioned "strange likenesses." Dumbledore theorizes that Voldemort transferred his powers to Harry in the original attack and Harry balks at the idea with this line.
For the next five books, each mystery to be solved was something we watched for ways that the "bit of himself" manifested. Harry could speak Parseltongue and knew when Voldemort was near. His ability to see into Voldemort's thoughts came in very handy. And, of course, the payoff was that the "bit of himself" was actually Voldemort's soul.
3. "And at long last, Harry believed him."

It is quite the roller coaster ride to read Harry Potter and the Prisoner of Azkaban. Even if you were to take a smaller portion of the book, you could find an insane amount of drama in just a few chapters. The Shrieking Shack scenes are just that. Harry goes from violently attacking a mass murderer to trusting the man who had talked him into trusting him.
The moment on which so many stories hinge in the series is when Sirius tells the story of what he went through to protect Harry and Harry believes that he is sincere. Minutes later, Sirius risks his own safety to save Harry, Ron and Hermione from a loose werewolf. Two books later, he is killed while fighting to save Harry and his friends from the Death Eaters. I cry at "The Forest Again" every time because when Harry wonders what death is like, Sirius reassures him that it is "quicker and easier than falling asleep." Because of this choice to trust, Harry gains family that he hasn't had since his parents' deaths.
4. "And I--I shall act as I see fit."

This line, spoken close to the end of the book, is a full book before the chapter called "The Second War Begins," but that title could fir this moment as well. Voldemort has returned, Snape has exposed his Death Eater past in order to convince the Minister for Magic that the threat of war is real, and Cornelius Fudge calls it all insane.
This is what Dumbledore calls a parting of the ways. He promises and threatens to act as he sees fit and this leads to his ouster from the Wizengamot and the year-long campaign to first discredit and then arrest him for working against Voldemort. It is also the moment that inspires him to reform the Order of the Phoenix and this is the organization that will literally save the wizarding world by the end of the series. We don't like Fudge here, but his blindness paves the way for victory made possible by his opponents.
5. "NEITHER CAN LIVE WHILE THE OTHER SURVIVES..."

"The Lost Prophecy" is one of those chapters where there is so much to take in that it's hard to keep it straight. We get the story of how Sirius came to the Ministry of Magic as a response to Harry's profound grief and open rage. We also learn about Kreacher's treachery. This is also when we read the prophecy that made Voldemort hunt down the Potters and discover even more of the heavy burden that Harry carries.
We spend the next two books hoping that everything Harry learns will make him the survivor in this statement. In the end, he sacrifices himself because "Neither would live, neither could survive." And that breaks hearts in a completely different way.
6. "Severus...please..."

I theorized since Chapter 9 of Year 3 that Dumbledore had to die in the sixth book and Snape would be the one to kill him. I was right, but the context of this scene had to be note-perfect for it to work.
Snape heard Dumbledore pleading at the top of the Astronomy Tower and his emotional response was one of "revulsion and hatred etched in the harsh lines of his face." We don't know towards whom that revulsion and hatred is directed. It's crucial that we know in the next book that this was a task asked of him well in advance that he balked at for months. That crucial moment is the only way to give him credibility with Voldemort and it is that confidence that is crucial in ending the war.
7. "The true master of the Elder Wand was Draco Malfoy."

Upon reading this line in the final duel against Lord Voldemort, it was all that I could do to not shout, "MERLIN'S BEARD!" For one thing, it meant that Voldemort's defeat was contingent on a Vanishing Cabinet Peeves broke to create a diversion on Harry's behalf...five books earlier.
Draco being the last person to take rightful possession of the unbeatable wand meant that Voldemort, as always, had a fatal flaw. In Greek, the word hamartia means to miss the mark and Voldemort has missed the mark over and over. Most importantly, the Elder Wand cannot answer to a master who has not won it and that is what turns the tide of this extraordinary battle.