People are gonna hate me for this, but I liked the 6th book the most, which was with the heart-wrenching plot twist of
...
The reason why is that year 6 is when they finally do the switcheroo of Snape as the DotDA professor, while Slughorn, Dumbledore's pal, gets the Potions class. It's ironic how Harry Potter then found that class to be his favorite of the year, of course, in large part due to the mysterious book that provided Harry shortcuts in his exercises.
There's something about all these mysteries that engrosses me in the books, and Book number six had the most in my opinion.