Reading threads like this makes me realize just how many games are out there. It's mind boggling. No one could ever play or read everything! (well, maybe)
I would vote for:
Midnight CS: The most interesting setting ever. Tons of fluff and not too much crunch. Evocative, dark artwork and interwoven themes.
Masks of Nyarlathotep: Best campaign i've ever read. It's just fun, even if you don't play Cthulhu. There is information in the game that won't even come up during play, like background details of NPC's and how their personalities became the way they are.
Monsternomicon, Fiend Folio, Monsters of Faerun, MM3, Plansecape Monsters 3 by Monte Cook and Monster's Handbook for fav monster books. These are fun to read for inspiration, and IMO Monster's Handbook is just fun.
ARS MAGICA! Talk about a fun book. It takes magic to a whole new level. For instance, the chapter on acquiring a familiar takes several pages by itself, including forging the three cords, the traits the animal gains per season, and the intricate details of the friendship you are building for life. DnD takes a slightly different slant. But Ars isn't for everyone, although rules for character personality and background can be adapted to any game system.
Planescape: yeah, these were fun to read. Monte's monster book has the originals of tons of monsters that made it into 3e, like the Devourer, and his descriptions in that book are often 2 pages per monster.
Lots of game systems i've never had the chance to explore, mostly because of time and monetary contraints.