The most horrific, gut-wrenching, deeply-disturbing short tale I've read in my life?
Well, even 'after all these years,' I've got to write that it is D. Etchison's 'Daughter Of The Golden West.' In which a couple of college kids make the mistake of 'going too far,' whilst investigating the murder of their beloved classmate.
Suffice it to say that they meet up with, in among the most horrific endings in short-tale history, a few 'sisters.' Sisters, who, descended from from the Donner Party (members of which, due to potential starvation, had to resort to eating something, 'rather unpleasant'), have inherited a taste for...
Well, I'll let you get to the rest. In, The Dark Country
If you have not read this one, it is a treat, to state it mildly.
'Honorable Mentions' in most horrific short-story category - to me personally? Stephen King's 'A Good Marriage,' from Full Dark, No Stars. Charles L. Grant's 'If Damon Comes.' And, not to be dismissed lightly, Hawthorne's 'Young Goodman Brown.'