Right now my list for that category stands at 2 entries; I hope to be able to add to it in years to come.
- sylviavolk2000 (on AO3) - I went on about her Highlander fics at length last year, although now you can download her stuff in .mobi format for Kindle, if you're a fan of that. But I'll touch on why she surpasses her source material in my eyes: she writes non-modern characters very convincingly. And more than that, she writes old characters convincingly. (Like, millennia-old.) That's really hard to do, as the plethora of badly-written "immortal elves" can attest. This allows her to flesh out the world, characters, and relationships that are established by the show, though at times fleetingly, or with an uncertain hand. And unlike the show, which basically betrayed the viewers in s6 by turning into a series of audition reels, she finishes developing the important relationships and then gives them fairly satisfying conclusions. And yet, I'd argue that her fics are more accessible to a non-fannish reader, not less; her stories are generally complete unto themselves, and long enough to really sink your teeth into. I can't begrudge her having ended her Highlander fic-writing days, as she's left us with a comfortably-large shelf of fics to enjoy.
- miss_porcupine (domarizione on AO3) - As most of you probably know, I used to be heavily into the Stargate: Atlantis fandom (and it's still fun to visit from time to time). But I'll freely admit that the show's writing lacked ... an awareness of logistics, let's say. Not the case here--plenty of logistics to go around, as well as an exploration of the military side of the Atlantis expedition--and the character of John Sheppard. The way she writes the expedition and the characters and worlds they interact with, you can believe it as an actual going venture. Some of her longer stories would've made great episodes for the show that could've been but never quite was, and she's created some of my favorite recurring OCs, probably because they don't feel like OCs, just characters from the show who never quite made it onto camera. And again, even though she's done with the fandom, there's still a lot of fic to enjoy. I keep refinding fics I'd forgotten about--not because they weren't enjoyable, but there's just so many of them!