I like discovering, or leading players to the weird demented things going on in the dark corners od the adventuring realms. Basically DCC’s wheelhouse. So DnD stuff like the Yuan-ti I love. But they’re relatively fixed, once you know them, you basically know them, they can get up to all kinds of messed up stuff, but it does have a pretty singular goal.
So I’ll nominate Kua-Toa as the best of DnD’s dark corners of the realm creations. The idea that their collective subconscious can manifest minor gods is a brilliant idea. Plus they crazy and demented, so isolated bands of them can get up to who knows what depending upon unique past and environment. It allows each isolated band of them to have their own particular messed up culture and religion with an in-built boss fight with a god-creature of whatever powers you can imagine.