Ehhh, everyone else has already said 1e DMG, so I'll pick something else just to be contrary. The 1e Fiend Folio. Some of the most amazing and flavorful monsters ever (Githyanki, Drow, Death Knight, Flail Snail, Hook Horror, Slaadi). Sure, some of them had appeared in adventures or periodicals, but for many of us who didn't follow those closely, the FF was our first exposure to monsters as compelling as those derived from mythology. Really changed the way that the people I knew thought about monsters. Great book.