What genre is that? I assume you’d say (children’s) fantasy, which they certainly are, but that’s not the only genre they fall into: they’re also school stories, and to some extent mysteries, which may help to explain their popularity. (I know at least one adult fan of the books who, in general, doesn’t much like fantasy but does love mysteries.)
True, there are other books that are better, in various ways—more original, more flawless, more profound, more beautifully written, etc. But it makes perfect sense to me that the Harry Potter books are as popular as they are. They’re fun to read, with plenty of humor and suspense and warmth. They depict a world that kids (and adults) would love to inhabit, but with enough danger and conflict and pain to keep the stories interesting. They’re relatively easy to read and follow: they don’t place great demands on the reader, though they do get more complex and mature as the series progresses. And they have enough detail to stand up to re-reading.