With all credit to reddit user /u/ragan651 via their post here: https://www.reddit.com/r/XFiles/comments/33zhcr/the_essential_xfiles_episode_list/
Summarizing their commentary on this curated list:
These are not the best episodes, but the ones most important for catching up with the X-Files mythology.
It's a long series with a complicated plot, and only some of the episodes are about that plot. So for those wanting to rewatch the show but not sift through the entire series, these are the essential episodes of The X-Files.
This list contains two types of shows:
- ones that are not essential mythology episodes, but connect to the mythology
- ones that are the best episodes of the series, but not part of the mythology.
The first movie takes place between seasons five and six, and is important.
The second movie takes place after season 9, and addresses the ending of the original finale, and has some small connections to season 10.