oh man, I’d have to do some searching. However, if you don’t mind generalizations, I can say this:
The MCU fandom produces some of the absolute worst shit. Also, 50 Shades of Grey was a fanfic for Twilight, originally, that she tweaked for publication for the general audience. And it is shit.
The Sherlock (BBC) fandom produces some of the best.
I’m talking the entire range of critique, from grammar/spelling all the way to plotting and characters and style.
The trouble with recommending fanfic is that in order to appreciate it, you kind of have to know at least the overall plot and history of the show/game/movie/universe, as well as the characters. There’s not a lot of exposition going on in the fan works themselves. So if you aren’t above checking it out, I recommend choosing a fandom you know something about or really like then visiting Archive of Our Own and giving a few things a glance over.
Also, if you think there couldn’t possibly be fanfiction for the fandom you choose…you’re wrong.
be warned. Fandom and fanfiction have their own lingo that can be confusing. I don’t even know a lot of it. Also, not all of it is stereotypical slash. You can find a bunch with no shipping (romantic relationships) at all, if you want.
as a side note, I find fandoms fascinating from a sociological perspective. Groups form, there’s infighting and wars, it’s all very tribal…and most people don’t even know about it.
Also, be prepared for the scope. there are over 400k works in the Marvel fandom alone.