Yeah, I think the overall conditions of the "battle" have to be taken into account. A whole lot of Tommies didn't have it so rough on the first day of the first battle of the Somme, they were just mowed down and killed right away so they didn't have long to suffer. One soldier doesn't care that much about total casualties as long as he isn't one of them. The overall scope doesn't worry him either, he only cares about the volume of machine guns, rifles, spears, arrows or whatever directly ahead of him.
That idea of living for weeks or months on end among rotting bodies and human sh*t and the sights and smell accompanying those would have to account for "worst possible battle". E.B. Sledge describes some scenes in the battle for Okinawa that bring that idea to life.
To that end those trenches in WWI had to be among the worst conditions to live in ever. With honorable mention to the whole damn Russian front in WWII.
