Holy Thread Resurrection Batman!!!!
Besides the Core Books....
Shackled City AP Hardcover
PHBII
Expanded Psionics Handbook
Frostburn
Stormwrack
Updated (of current/semi-current stuff)...
1) Pathfinder RPG Core Rules - The more I read, the more im hooked.
2) Shackled City AP HC - This one is still hard to top as far as a single volume goes.
3) Castle Whiterock - Raising the bar on the classic "boxed set".
4) Pathfinder Bestiary - awesome compilation in here.
5) Necromancer's Tome of Horrors - still seeing references from here in some newer products. Great monster book, also better than anything post 3.5 MMI and the FF from WotC.
The above is based on what I currently still own. Adding older stuff, I would include stuff like ...
D&D Basic set - Erol Otus cover, pink books iirc. Long time ago so my memory is fuzzy. I still have the dice from this, they are orange with worn off crayoned numbers.

AD&D Player's Handbook - Obvious choice.
The Forgotten Realms Boxed set - The original grey box.
Temple of Elemental Evil Compilation - some of my fondest gaming memories.