Tough one. I'd have to say my 1e Deities & Demigods in all its copyright-shmopyright glory, with Cthulhu and Elric included. But my favorite thing about it is that right there next to Lolth, in thick black marker, I calculated the experience points the PCs got for killing her. Got to love middle school.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.