1e's fab 5 (players handbook, monster manual, fiend folio, monster manual II and definitely the DMG).
The Dragonlance novels. As a kid, I loved these.
Dungeon! the board game. Fun in itself, plus I raided it for its miniatures.
B/X - Moldvay/Cook rules. My favorite.
Rules Cyclopedia - But this is a very complete version, a done in one that can last forever, if you remember to give Thieves and Mystics +30% to everything.
2nd ed. The settings. The beautiful beautiful settings.
3rd ed:
Draconomicon - finally fixes a pain in the butt in 3rd ed -- actually puts together dragon stats (including spell selection) for all age categories for the 10 basic types. A big timesaver.
PHB II - goes a long way to helping the fighter, and has need class options for others like the Hexblade. Also nice quick NPC rules in the back.
Expanded Psionics Handbook - finally gets the balance right.
Complete Arcane for the Warlock. A class that is not only flavourful, but easy to use for players and DMs alike.
Tome of Battle - Like, wow!
Mutants and Masterminds - very well done for comic book supers.
Testament - I actually learned something about the history of the Jewish people from this game.

In addition to combat, has nice "here's your community, how do you keep it going in the face of various troubles?" rules.
Iron Heroes - Lets you be METAL without magic items.
Castles and Crusades - A simpler version of the game. Great for new players too, IME.
Book of the Righteous - A very nice pantheon and background. I have a friend that swears by it. Pity it wasn't updated to 3.5.
Kingdoms of Kalamar Campaign setting - Personally, I found it very well done and with lots of plot hooks waiting to grab hold. But this one varies from person to person.
Pathfinder Core Rulebook & Bestiary - Keeping the Flame going, and doing a fine job of it. Kudos for putting out a Pathfinder SRD!
4e - DMG - some good advice in this one.