The Trollpak supplement for RuneQuest blew my mind back in the day. I haven't seen anything before or since that did so to quite the same degree. Taking a faceless "enemy race" and explaining in astounding depth "why they're this way, naughty word's way more complicated than is dreamt of in your philosophy." These are not faceless orcs for the slaughter like D&D, this is a rich (if brutal) culture than goes back way further than your own. It was eye-opening to 12-year-old me. It forever changed how I thought of RPGs, and in particular, species in RPGs.