Enjoyable stories under 100 upvotes, over 15k words. If you're browsing my favorites for good stuff, start here and give littler-known authors some eyeballs.
Novel premises, worldbuilding, writing styles, etc.: A showcase of the diversity of fanfic, and for ease of later reference when I want to cite stories that have done X or done Y especially well.
Stories that offer commendably grown-up takes on sex, sexuality, and relationships: heartwarming respect, consent awareness, kink positivity, open and honest confrontation of taboos, etc.