The Deathly Hallows, of course.
I mean don't get me wrong, Horcruxes are fascinating as well, but I absolutely love the Hallows. Not just from The Tale of the Three Brothers story but how it was incorporated in the books and how well it was all connected. Also, there's something about the morality tale of the Hallows and what it means that is significantly different than allowing yourself to place pieces of your soul into objects, thereby being forever immortal. But at the same time, the genius way that JKR used both in the books to complete the story always astounds me because it makes you just rethink, and reread, the previous books to put it all together and it's just fascinating.
Rest of the 30 Days of Harry Potter Challenge.