HERO core rulebook- pick almost any edition- has to get my nod just for sheer versatility.
But there have been soooo many other books I consider to be true gems that shaped me as a gamer:
Original Traveler 0-9
Central Casting
The Primal Order series
Green Ronin’s Book of the Righteous
The Fantasy Trip/In the Labyrinth
Space:1889
RIFTS (mainly for the fluff & the setting, not the mechanics) & some of its sourcebooks
Deadlands
SPI Universe
D&D Core 3.5, along with its close cousins- Pathfinder, Midnight 2Ed, Arcana Evolved, FantasyCraft, True 20, etc.
Mutants & Masterminds 2Ed
WoD/NWoD
Scion Hero/Demigod/God
And while I don’t have a particular love for the game, GURPS 2Ed is important enough to me that I bought a copy of the core rulebook (and the martial arts supplement). I was in a group that played a lot of it, including some playtests, like GURPS: Vampire. I had a lot of fun in those games, and made some particularly memorable characters.