I grew up with the Harry Potter series and loved it as a kid, but I can't pretend it's actually good writing. It's full of all kinds of plot holes, the wizarding universe doesn't make a lot of sense, there's thinly veiled bigotry (particularly prominent in the naming of characters), and Rowling as a person is all sorts of problematic that I can't endorse giving money to.
But Harry Potter and the Methods of Rationality, or HPMOR for short, is a really unique and fascinating full length fan work for any fan of Harry Potter (and also anyone who read Harry Potter and just didn't get the fuss). Though you don't actually need to have read Harry Potter to enjoy HPMOR, as it mostly reuses characters and settings but is otherwise entirely original.
The premise is simple: what if Harry was actually smart? In fact, not just smart, but a child genius. The wizarding universe as a setting with all its flaws isn't changed, but Harry recognizes those flaws and takes advantage of them. You can think of this book like someone very smart nitpicked over the Harry Potter universe trying to identify everything that doesn't make sense or would be "overpowered". It admittedly does often have "I am very smart" vibes that can sometimes get a little cringeworthy, but it's overall extremely entertaining and a one of a kind work that I wish I could reexperience for the first time.
It's also not just Harry being smart, because that would be boring if it was just him. Voldemort is also recharacterized to be much more competent and intelligent. This sets up interesting conflicts that I won't elaborate on to avoid spoilers, but I can say that the way Voldemort is characterized is brilliant and far better than the actual Harry Potter books.
Overall, it's one of the most entertaining books I've ever read and a fascinating critique of sorts of the original Harry Potter series that many of us grew up with.