I've run two long-term campaigns since
GURPS Fourth Edition came into being:
The Company: The "kit" for my current secret-agents campaign is the Basic Set (both volumes), High-Tech (for the gear), and Martial Arts (for the fighting) – with occasional use of Power-Ups 2: Perks, Power-Ups 3: Talents, and Tactical Shooting. I keep Social Engineering on standby, but so much of what goes on is deeply clandestine and completely avoids humans that this fine work has yet to become a core tool. This campaign inspired the Action series, but those supplements are really more "my interpretation of the above stuff, put into writing"; I'm not using them per se.
The Dawn of Magic: The "kit" for my previous fantasy campaign was the Basic Set (both volumes), Low-Tech (for the gear), Magic and Thaumatology (for the magic), and Martial Arts (for the fighting) – with occasional use of Mass Combat and Powers. This campaign informed the Dungeon Fantasy series, but here, too, the supplements were notes on my general approach rather than materials I used. Similarly, many concepts eventually found their way to Power-Ups 5: Impulse Buys and Thaumatology: Magical Styles, but I didn't use those works.
Analyzing that, I guess it would be fair to say that my most basic tools are the
Basic Set,
Martial Arts, and the
Power-Ups series, and that I'm highly prone to using the
-Tech books and genre quick-starts.