I'm going to have to go with FC: I, just not enough was useful in my game, the demon archtypes was something I was basically using already, and the information about all the layers was interesting, it wasn't really usable. Unless you are the poster directly above me,
how often are you running/playing in the abyss? Meanwhile the FC:II had a lot of immediatly useful stuff, and more suggestions on how devils act on the prime, and easy tie-ins to devilish politics between factions.
All that said, I am very glad these three ended up on top. They would have gotten my votes for the best three from the original list. I may have to go for the PHB2 for the win, some of it was useless, but some of it has had profound impacts on my game.