lulz aside, it's verging on the absurd to try to quantify this.
Had it the worst, as in - quantity? Because, any one human can only be killed once. Any one human can witness the murder of their closest family only once per member. Once you murdered someone's entire family, then perhaps proceeded to violate the last survivor, then torture, then kill, that's pretty much it, that's about as bad a day as someone can possibly live through, unless you do the violation with samurai swords up the butt, like some people in Nanking had to experience.
If you want to see who has had it worst, in terms of the above multiplied, then it becomes a bit biased, since large populations will be more numerous than small ones, but then again, in the smaller ones, the comparative impact is much larger.
While relatively small in numbers compared to the big famous events, situations in southern Darfur - or the short lasting massacre/ethnic cleansing of Georgians in Abkhazia, are absolutely nightmarish in terms of individual human stories (read up on it), worst of the worst (except for the samurai swords up the butt), but completely dwarfed in terms of *number of victims* when compared to more "historically significant" events