There are soooo many Fanfictions out there, so how do you know what is worth reading? Simple. Create a rec list! If you have created one yourself please post the links. If not simply post theme + title + site here!
Romance:
L.J. Smith- Chimera
http://www.fanfiction.net/s/360519/1/ Inuyasha - Captive Soul
http://www.fanfiction.net/s/1308711/1/ Naruto - Loophole
http://www.fanfiction.net/s/2430922/1/ Humor:
Inuyasha - Puppy 101
http://www.fanfiction.net/s/2240788/1/ Inuyasha - Taisho's Darling
http://www.fanfiction.net/s/1774880/1 Dark:
Inuyasha - Moonstruck
http://www.fanfiction.net/s/1490981/1/ Harry Potter - Abyss
Harry Potter- Ascent (sequel to Abyss)
While Abyss is very veeery dark with more or less detailed violence, even crossing the line I can stand with rape, it is suspenseful all the way through. All the emotions (or anti-emotions) in this story will stay with you for a long time. Ascent is not that bad. I recommend to read it immediately after Abyss, because the feel of the first part makes it even better.
Fav authors:
http://www.fanfiction.net/u/75251/ (Ravyn; Kenshin AU; Oneshots)
http://www.fanfiction.net/u/839806/ (BelleEve; mostly fairytale like adaptions)
http://www.fanfiction.net/u/167799/ (Harry Potter; Draco/Hermine)