Zardnaar
Legend
The first boxed set was amazing with cool sorcerer-kings and The Dragon. Then the novels immediately messed with core parts of that boxed set and the modules followed suit. If you stick with the core boxed set and rules supplements like the monstrous compendiums and the splats you have a lot of cool aspects to draw from. Dragon Kings has a lot of great stuff for high level D&D. My magic user was happy to get his hands on protection from time and improved haste.
One drawback of the core setting though is that it is specifically a different power level from base D&D. Stats going into the 20s, and everybody a wild talent makes it hard to mix and match in a balanced fashion for say a ravenloft game that draws characters from different worlds (not that Dark Sun characters would generally be a good mix for gothic horror, despite the Kalidnay(?) Dark Sun realm).
Ravenloft scary? Lol. Try being stuck in the wastes under the dark sun, waters running out and there's a circle of ash on the ground....