My recommendations are purely selfish, of course, and heavily dependent upon the campaign that I'm currently running...
1. Pathfinder Core Rulebook - my currently-favored campaign ruleset;
2. Pathfinder Advanced Player's Guide - for my players to tinker with;
3. Pathfinder Bestiary - for me (the DM) to tinker with;
4. Shackled City Adventure Path - our base campaign, Cauldron as a backdrop;
5. Savage Tide Adventure Path - providing customizations and possibly a different endgame to the SC campaign, depending upon what the players do;
6. Ptolus - providing custom details of the continent's largest city;
7. Heroes of Horror - adding rules and surprises
8. Book of Vile Darkness - adding even more rules and surprises
9. Magic Item Compendium - adding unusual items for ever-greedy PC's
10. Draconomicon - providing details and "color" for the various dragons that the PC's have, and will, encounter.
I'd need these for our game to survive the apocalypse.
J.