The Shadow of Yserbius and the Fates of Twinion were my first experience with MUDs/Proto-MMOs. My dad, brother, and I all played on Sierra's INN/BBS type service. But the best part was the games could be played solo as well as on-line. My next experience with MUDs would be the Avatar mud via the Columbus, Oh Public Library's freenet service.
These were formative online gaming from back in the day. Also Legend of the Red Dragon and PLANETS.