Ask me this question a year ago and I would've said the LGG without hesitation, however in recent times I've found that the LGG just isn't enough, I want it all!
Currently Ivid the Undying is my primary source of info but that's only because I want to focus a campaign based in that area. I LOVE Carl Sargent's work and care less what all the die-hard's say about him.
But I've also begun collecting older GH material and am pouring over it all so I really don't have a primary source.
That said, I use the LGG as a yardstick, in more ways than one. All the gaps in it I fill and I use it as the template for my overall gaming in GH. But I also use it as a template for other material to measure up to. I care less what all the old timers say about how these books are crap, to me the LGG is what everything else should compare to, not the other way around. I came to that conclusion through my research and knowledge of writing, not through some elitist, over-weening, philosophy that just because Gary didn't stamp it with his approval, that it doesn't stack up.