In my opinion, it's both the best YA series ever and the best 판타지 series ever. Why? Harry Potter is the best of everything - the best characters, the best plots, the best writing.
The characters are people 당신 can fall in 사랑 with, and not just the main 히어로즈 and the main villains. Even 더 많이 minor characters are pretty extensively developed, like Snape, Pettigrew, Neville, even Dean 또는 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.