I think the only fic that has ever punched me hard enough in the gut to make me physically shy away is pwcorgigirl's Afterlife.

And it's not House's death at the end that does it -- it's the vision of a once-vital House, reduced to a brain-damaged shell of his former self, that just sinks its claws into my heart and won't let go.

I literally have trouble rereading this story. The only other story I can think of that I have such a strong reaction to is deelaundry's Locked Up and Set Free.