All the supplements are great, but this one sticks out to me quite a lot. Having a human option in a supernatural game, while feeling like they aren't underpowered or overpowered is a rarity, but beyond that, the lore in this supplement makes it a must-have in my opinion. It expands the world, yet brings it all into a cohesive whole, and shows how each part interacts with another. It's wonderful, and you can take it as it is, or use it to build on your own lore for your own setting as a jumping off point.
Yes, this one is my favorite, how could you tell? :D