Yes, I know it's a question that's very subjective. Still, it doesn't invalidate the question.
I have two favorites, for very different reasons.
The Teratic Tome, because it is amazing to behold (both in print and PDF). The unspoken setting behind many of the creatures presented must be brutally amazing. Did I mention the art is awesome and there are so many creatures I want to find ways to sneak in Rappan Athuk?
The second is the Tome of Adventure Design. I really should do a post a week, creating adventure locations and the like using the table in this book. The amount of inspiration in this book is beyond my ability to quantify - lots doesn't come close. Get it in dead tree format if you get the chance - there is nothing like flipping through this book to make you want to run a game session.
Now, despite my two picks above, I am sure there are numerous other choices that are just as worthy, if not more.
What are your picks?