I'm making my comment here because it's the most up to date version, even though I played the 2000 release version. Calendra's Cistern really stood out to me as the first FM I played that was on the level of polish that I would believe you if you said it was made by the official LGS team. The mission area is huge and lasted a good 2-4 hours for me. The map design was great and had a lot of really enjoyable vertical segments as well as hidden passageways. The story was pretty decent as well, and contributed to the feeling of polish. My favorite feature of this FM was the in-game shop at the start, where you could spend gold you looted in the mission to purchase tools. This is the sort of gameplay that is "show, not tell" where you get to interact with the game in order to gain tools rather than the static pre-mission screen that you just plan out the tools you start with. Currently, I hold this mission as my gold standard for overall quality that an ideal FM should strive for to equal or surpass missions from the original campaign.