I'm a huge fan of Goblet of fire because I like the Quidditch world cup beginning and the Tri Wizard tournament is a great plot device. I really like Half Blood prince with all the detective work that Harry and Dumbledore do. However any book that has scenes or chapter set outside of Hogwarts are great fun to read such as the visits to St Mungos in the Order of the Phoenix. I like reading about the wider magical world.