[whswhs]

Well, leaving out the core rules books, I'm going to pick the following:

GURPS Alternate Earths 2: It has a variety of really imaginative timelines, more interesting than those in GURPS Alternate Earths 1, and with a lot of texture.

GURPS Powers: The single most useful of the major system expansion books, with its extremely detailed treatment of special abilities, helpful for everything from supers campaigns to "magic as powers." The concept of power modifiers is a really great unifying mechanism.

GURPS Power-Ups 2: Perks: It's hard to choose just one of this series, but I consult this constantly in creating new characters, and I'd find it harder to get by with just the perks in the Basic Set than with just the quirks.