I have respect for every one of these. They all are extremely successful, and I think it's ignorant to say that there is a best one. At this point it's extremely subjective. There will be fans of each series that think it's the best. Very few people can really be subjective and take a look at all the different aspects of these novels and compare. And really, I don't think its fair of the literature to do that. So, I have mutual respect for all of them.
But I do have a favorite, of course, and it's Harry Potter! I've read a lot of dystopians, fantasies, etc. and nothing ever comes close to my Harry Potter. It was my first big series that I've ever read and it has strong connections with my grandma (we fan-girled over every book and always had a special day together when the new book or movie came out). I always get the feels when I read the series again (I'm on my 12th time rereading the whole series). I always feel the exact same feelings in the exact same parts no matter how often I read them! It's just one of those books that have had a huge impact on my life and my interests.
I think that the series is wonderful, the characters feel alive and complex, the plot is exceptionally interesting in all of her books (even if they follow things that have already been written before in a different way). The thing J. K. has going against her is time. If she had jumped on it sooner (and was born sooner) I think she'd have a lot more respect than she does today. But it's not like she has plagiarized anything, so I can't penalize her writings for coming later to the game and being technically less unique/original than the others. She took now common ideas and twisted them into her own unique story, adding a lot of creativity and imagination forming her world. Her world is very vivid, partially because she made the smart choice of combining muggle with wizard world, so it really feels immersive and real.
That's one big issue I have with other fantasy novels is that they sometimes feel too far fetched, like they could never ever possibly be real. Harry Potter feels realest to me. I have major props for Tolkien and his middle earth, because that took a lot of work and imagination to make something that massive, but I could never get into it or feel like it was "real". And I didn't like his characters nearly as much, they seemed boring and bland. But I still loved Lord of the Rings, and it is just as good technically (if not better) than Harry Potter.