My library of MGT is admittedly small: Core, Darrians, Dynasty, and Animal Encounters. The first two sate my lust for interesting information to build a game with. The second two are tools for me to use; things to add interesting wrinkles to a game. While they are impressive, they aren't the kind of thing that jumpstarts my imagination right off the bat.
So I voted for the core rulebook and the Darrians.