In my opinion, it's both the best YA series ever and the best Фэнтези series ever. Why? Harry Potter is the best of everything - the best characters, the best plots, the best writing.
The characters are people Ты can fall in Любовь with, and not just the main Герои and the main villains. Even еще minor characters are pretty extensively developed, like Snape, Pettigrew, Neville, even Dean или Seamus. They all have histories and quirks and unique personalities and very few authors are able to so vividly create such a huge cast of characters.