The "green book" Historical Reference series are favorites of mine.
I always loved D&D played in a pseudo-historical setting. I've long been sad that WotC has stepped more and more away from that over the years. AD&D 1e and 2e always seemed to be "Medieval Europe with Tolkien influences and magic and monsters added", which seems about the right default tone for D&D to me. 3e went into Dungeonpunk and pseudo-historical games and the idea that D&D had any roots in historic settings and culture showed up in some early 3e era Dragon articles, but faded quickly. I ran so many games set in the Roman Empire, or during the Crusades, or among Vikings with those books.
The brown splatbooks were very hit-or-miss. The Complete Book of Elves was cool, but full of broken, cheesy stuff. The Complete Priest's Handbook was pretty much a waste, it was more a DM's guide on creating religions and priesthoods for games with very little for players or DM's who were using an established setting.
I remember Complete Ninja's Handbook being the closest we really got to a 2nd Edition version of Oriental Adventures.
The Dark Blue/Grey covered DM's handbooks were good. Sage and Specialists basically introduced the first implementation of the NPC class concept that would be a core part of 3e. The Castle Guide was a great book, and was definitely also rooted in the idea of D&D as pseudo-historic gaming, as it was basically how to implement historically accurate castles into a D&D game and the society that would function around such a castle.