Unfortunately, I haven't really read past Volume 11 (I've only been able to go with the Dark Horse releases and the translation of Volume 11), but the saddest moment from what I've actually read had to be when the Hawks find Griffith in the dungeon. I mean, this has been stated before, but taking a previous volume and comparing it to this particular scene is quite disheartening. Even so, it was mostly his body that was terribly disturbing. While his face was covered by the mask for the most part, its outline seemed relatively plump and healthy
(But that's an opinion for another thread).
Coming in close second is a related scene, with Guts' one-on-one conversation with Griffith in the carriage in Volume 11. I just thought it was sad when Griffith tried to pick up the sword, but wasn't able to due to the results of his torture.