You can see when she became big enough to say no to her editor.
The first one is good, crimping Roald Dahls style, telling a cute little power-fantasy story.
By the Fifth book she's really far up her own ass.

edit: It's been much longer since I've read 2 and 3, but I remember them as being pretty decent. I've read the first one a bunch of times because I grew up with it, so it's good for when I wanna practice another language. I think I've read 2 and 3 2 or 3 times, because reading the first one has made me want to go on a nostalgia binge (this is like 8 or 9 years ago, be nice). I haven't read any of the other ones more than once, but I remember being very disappointed with the movie version of the fourth book because it didn't really dedicate a lot of time to the quidditch thing a good part of the book is about. Past that is a complete mismatch. I remember the bus scene from the fifth movie because it's a cool idea and I remember the sixth one having Snape in a book and the seventh one having Ron be worried he's going to get cucked or something.