Spider-Man: Into the Spider-Verse' introduced Gwen Stacy and Miles Morales in a way canon never really got to explore, so the fanfiction scene exploded with potential. If we're talking absolute essentials, 'Echoes of Us' on Archive of Our Own is basically required reading. It picks up after the first movie, deals with the mechanics of dimension-hopping in a way that feels logical and heartbreaking, and has this aching slow burn because of course they can't just be together. The author nails Gwen's guilt and Miles's longing without making either of them pathetic. There's a later section involving their future selves meeting their past selves that messed me up for days.
For something with a completely different tone, 'Friendly Neighborhood Spidersona' is a hilarious series of one-shots and short arcs set in a universe where they both ended up in Miles's world for good. It's lighter, full of Brooklyn bodega runs and awkward team-ups with Peter B. Parker, but it never loses the core of their bond. The banter is top-tier, feeling very true to the movie's vibe. I go back to it when the angstier stuff gets too heavy.
Honestly, the tag is so massive now that quality varies wildly. My advice is to sort by kudos or bookmarks and be willing to sift. Some hidden gems are the more speculative AUs—I read one where Gwen was a radio DJ and Miles was a graffiti artist in a non-superhero setting that captured their dynamic perfectly through just their voices and art. Sometimes the best stories aren't the ones that stick rigidly to the movies.