When it comes to Tolkien, I don't think anyone will argue that he has the inferior writing style. The man's prose is one of the best ever encountered in the English language, where as Rowling isn't nearly as good - one of the reasons that people have rebelled, visually, against the HP movies but most praised LotR for costumes and settings. Even people who disliked the films.
Choosing a favourite Potter book is hard. I would immediately discount the first two, and the fourth. The first two were very good children's books, but I don't personally think that they reached a level one could consider "excelling" in literature. They are still quite rough in their writing types and you can tell a lot of the finer points hadn't been sharpened yet in her book of the books. The fourth was the most bloated of the series. I think she took her luxury of writing and expanded on a lot of things that really didn't need to be expanded on.
I rather enjoyed the fifth book when it came out. It wasn't my favourite, but it was very, very good. I thought Harry's evolution into an angry 15 year old was very natural. I remember being 15. I didn't have Dementors and Voldemorts to fight, I just had to listen to Limp Bizkit. I agree with Shades: the battle at the end was one of the most climactic moments in the series, and it really gave you the impression that Harry was growing up. In the first few books, Harry's survival was requisite on his unique background and his quality of character. But in the fifth book, he learned to fight, and put it to good use. It really gave a new atmosphere. I enjoyed it.
The seventh book is a very thorough wrapping-up of the novel. I am not sure, to this day, if I loved it. But it was very good, and it didn't leave me feeling empty as many series-enders have. Some of the moments were very adult indeed, and I liked that. I also liked how she finally delved into the further past, something I had been wanting since book one - and that is the bit that has left me wanting more. I like the fictional history of worlds (a very, very good reason to love Tolkien as I do!).
The third book was my favourite for a long time, it is easily the best written of the shorter books, introduces two of my favourite characters in the series (Lupin and Sirius), and swings the children's attention away from their childhood friends (Hagrid is the big one, his role is very much minimized in this book as he is replaced as Harry's older mentor figure - first by Lupin, then Sirius, then finally Dumbledore). I think it has a darker tone, and it shows us some of the inner workings of Snape too.
But the sixth book has really become my favourite over the past few months as I re-read the books in a row for the first time. I think it is the best written of the novels. Deathly Hallows has a rushed feeling in places - an army feel I called it, "hurry up and wait". HBP is very nicely paced, and introduces the mystery of the title character. I rather like Slughorn as well, he has quickly become one of my favourite teachers. But I love the way the book shows us the mind of Dumbledore, and how he has pondered the enigma of Riddle for decades. The memory gimmick is amazing - the scenes where Dumbledore pieces together Voldemort's past bit by bit have drawn me back again and again. The rest of the drama I can almost do without - but those four chapters...amazing.
Yeah. I wrote a lot