While this has always been my favourite Harry Potter book, as it is at its core the start of the main plot that carries through to the last book. This time everything feels bigger and more dangerous. It also introduced the idea that sometimes nothing can be done about a situation and that sometimes the government is useless. These themes are very important to not only the story but are good life lessons as well.
As always the characters are fully developed and seem to have left their annoying second-year phase. They are more like the characters that we will see for the rest of the series. The new characters that are introduced seem to have a greater depth to them than the previous ones (though that might be because they are sticking around for a bit longer).
As I mentioned earlier, the plot in this book is a lot bigger and the stakes seem a lot higher and a lot more personal. I also think that the challenges in this are a lot more relatable as they are more personal to Harry as opposed to a general threat to Hogwarts or the Wizarding World.