Mystara is the bestest D&D setting.
IIRC, it started around 1981 with the Moldvay/Cook Expert set. It was developed a little bit further in 1983 with the Mentzer BECMI line. Most of the BECMI series of modules were set there. In 1987 it received Gazetteers, which were a new thing at the time, for many of the countries listed. The line ran mostly from 1987-1989 and largely replaced modules as the published material. There was a smattering of material published after 2e came out and some of the material was converted to 2e. In 1992, I think, there was a reboot of sorts with a series of boxed sets.
The best things about the setting, I think, are its history and connection to some of the earliest forms of D&D and a relatively coherent set of supplements. Some setting like Forgotten Realms are drowning in material and, to me, lack coherence in the setting. Others, like Birthright, have a tight focus but lack the depth of module support.
You can get almost all of the B and X series of modules on dndclassics or ebay for not much money. Most of the Gazetteers are available on dndclassics as well, but they are pricey on ebay. In addition, the people at pandius.com have developed a massive amount of high quality fan made material. The maps by Thorfinn Tait are worth a visit all by themselves.