Harry Potter is pretty much the only fandom that I read fan-fiction in. I have checked out a few Dresden Files fanfics, but have never really gotten into them, not wanting to spoil the books for me, which I haven't read as of yet.

 

In no particular order, here are my top 5 fandoms. I'm afraid that I can't quite boil down why I like each one, since these are my tops not because of how much I like the universes themselves, but how good their stories are (and for this reason, I instead offer a few stories from each).

Harry Potter Principally Prince of the Dark Kingdom; Harry Potter and the Wastelands of Time; and His Own Man.

Star Wars Principally Attachments; Drive You Mild; and Greyspace. I'd also have to suggest pretty much anything by Corana.

Firefly/Serenity Principally Forward; Green Eyes, Browncoat; and The Man with No Name. Firefly is especially suitable for crossovers, for some reason.

Avatar: the Last Airbender Principally Embers; Where Puppets Stand; and As It Should Be.

Dead Like Me Principally Reaping the Whirlwind (Boom Boom Ba Remix); Fear; and Having a Lousy Time, Wish You Were Dead.

Invader ZIM and Doctor Who nearly made it in here, but alas....

And Jedi Buttercup has written a small load of brilliant crossover drabbles as well.

 

Harry Potter and Naruto are probably my two top fandoms.

A distant third would be Batman/DC, and after that The Dresden Files and Bleach to round off my top five.

 

1-Harry Potter-Depth of fandom and skill of writers in it(mostly the ones on DLP)
2.Kingdom Hearts-Awesome game(Though I have yet to find a single good fic for it. Just about all of them involve HOT BUTSECKS)
3.Artemis Fowl-Awesome books, but no good fanfics for it.
4.Dune-Favorite book, but nearly impossible to make a fanfic out of(because much of he book is politics on a galactic scale).
5.His Dark Materials-Awesome book, but I haven't found lots of fanfics for it.

 

1: Harry Potter is Harry Potter. I trust I've made my point.
2: Dresden Files The actual series I like better than Harry Potter, but it just doesn't have the following of the above.
3: Heroes is a taste I've recently acquired, but one I enjoy quite a bit. Acceptable heroes fics are not nearly as hard to find as those in other fandoms.
4: Mass Effect Shepard is a badass, and the game is so filled with choices and divergences that it makes for an interesting set of stories.
5: Code Geass The best anime I've ever seen, and since in fanfics the main character often does not obsess for 20 minutes over who his opponent is revealed to be, It's easier to enjoy than the anime.

 

1: Harry Potter - nuff said.

2: Naruto - Popular verse, loads of fics, the potential universe is great.

3: Bleach - Another popular shounen manga and anime. Surprisingly 'few' fics, but as an anime lover, this was a must.

4: Disgaea - Epic cult anime metagaming parody stuff. I honestly love the world, the major characters of the first game, and constantly wish there were more fics.

5: DnD rules universe - Includes actual DnD, Forgotten Realms, LotR, Dragonlance, that stuff - good oldschool fantasy. I really love seeing the magic aspec of these, since it's actually complex.

There are ALOT of minor books and animes that I've went thru, but that had a pathetic showing of fics. The first three are just so big that you're bound to find some goods fics.