I don't think I'm allowed to link NSFW stuff.
But the worst fanfic I've come across was not one that was poorly written- but came so far out of left field I actually couldn't finish it.
To put it in perspective- I've read all the crazy stuff from SAM, cheerliee's garden, cupcakes, rainbow factory (the big 4) and more without batting an eye. I do recall thinking 'that would have hurt' during a scene in cupcakes- but that's pretty much it.
I'm totally not ashamed to admit it got me. Maybe one day I'll finish it.