All the books were exceptionally well written, but to my personal liking The Order of the Phoenix takes the prize, like most of you guys. You actually get to see how screwed up Harry was after the Triwizard tournament, luna is one of my favorite characters in the series and now that i look back i see how well done she was as in introduction and development. Probably one of the most meaningful moments for the HP fandom is Sirius's death, followed shortly after by the prophecy. That sealed an awesome book, easily one of the best ive read.
As of movies, i just hate it when they differ from the books, and i was glad they didnt take out THAT much from The Deathly Hallows in the script, at least when compared to The Half-Blood Prince, Both Part 1 and Part 2 of the Deathly Hallows are my favorites.