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The Forgotten Realms Campaign Setting. With great artwork, layout, new rules, presentation, feats, prestige classes, and mountains of information about the setting, this is the way a campaign setting book should be done.
The 3.5 boxed set and Expanded Psionic Handbook do not come into consideration. They go first in any case. But that considered, if I should name only one after that I would say Grim Tales.
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Edit: Grim Tales is a d20 book, not a D&D book. If I had to keep only one book after the 4 basic books, I would opt either for Unearthed Arcana then create my own setting, or I would keep Dragonlance campaign setting. I just love that setting, or maybe it's just the book, but for some reason don't feel the desire to run it.
d20 D&D: Oriental Adventures and Manual of Planes
AD&D 2nd Edition: The City of Greyhawk Boxed Set (all time favourite) and Mostrous Arcana: Sea Devils
AD&D: The World of Greyhawk Boxed Set and The Dungeon Master Guide
OD&D: The Known World Gazetteers series, in special Glantri, Karameikos, and Shadow Elves