The darkest story I've ever written? :) Depends on your definition of dark, I suppose... I once wrote this thing that was entirely set in the armpit of the giant of midnight, who was stuck in a hole seventy bazillion leagues under the earth, in a land where the sun never rose, and...
Uhm. No, of course not. XD
I've written probably most of the darkest aspects of humanity, and these subjects tend to be something I gravitate towards both in my art and my writing. I'm always interested in the imperfections of humanity - the broken bits of our minds and ideas, as well as the broken parts of us as a whole - our societies and cultures, even our buildings and landscapes to some extent. It's what makes us imperfect that's so beautiful, in my opinion - and this is reflected in the style of my artwork, too, which tends towards choppy, textured life rather than glossy, plastic photorealism (which I can also do, if I am so inclined, but feels dead to me and, I fear, my audience too).
So yeah. With such a fascination for what you could generously term 'dark' subject matter, I guess I've pretty much done most of it... I've written rape and murder and torture, abuse and bullying, apocalyptic cities, slavery, the works... but I try to put an interesting spin on whatever I do. There's nothing interesting in the cliche, the idea everyone has... and everything interesting in looking at these things from a skewed point of view.
Probably one of my favourite characters to write, ever, was a sociopath named Finley. I wrote him on a public creative writing forum, so he got to interact with the characters of lots of other writers, which was fun. He wasn't a slick, smart, impossibly cool sociopath like you always see in movies and books (Silence of the Lambs, and practically every other horror movie starring a mass murderer ever), he was a regular sort of bloke who just had a complete inability to understand 'right' from 'wrong', making him the ultimate hedonist, but at the same time hopelessly alone and unable to ever satisfy his desire for fun and joy in his life, because he pretty much couldn't feel any positive emotion. (Most positive emotions stem from relations with other people - love, for example, requires that you are not an island). The fun thing about him was exploring our own reactions to morality... and discovering that morality is in inherently flexible idea anyway... that morals are always subjective. I delighted in showing my readers the parts of themselves that felt and acted the way he always did, because it bloody terrified them... and I got endless notes telling me how much they enjoyed reading him, despite the fact that he horrified them. During his time he killed a handful of people, and raped two others, and he always had a damn good reason. It wasn't a logical reason - the guy was crazy - but it made sense, to the readers. And that was what scared them, and made it fun for me. He was a very interesting challenge, and one that I enjoyed. :)