Who for, the attacker or the defender? Both battles are defensive victories- horrible as they were they achieved something, the men who suffered there did not do so to no purpose.
Whether that actually makes it feel worse, to know that even while you are happy you have frostbite because that way you can't feel the lice anymore, and starvation and sleep deprivation aren't that that bad as long as the hallucinations are interesting, that you are actually doing something that matters, that there is a purpose to what you are going through-
and that means you can't stop, there's going to be more- is that actually worse than if the battle is completely senseless?
To judge from survivors' accounts, actually, no; it is possible to find accounts from both winning sides that are actually highly positive in tone; look at some to the things the graduates of the Stalingrad Academy of Street-Fighting said later on, of some of the things the survivors of Verdun said between the wars-
there is a lot of pride to be found in some of them, pride in having endured, survived and to an extent triumphed in those man- made hells.
The defeated tend to tell a different story, admittedly, and I did vote "other", thinking of a battle that both sides effectively lost, where the pain and misery was overwhelmingly for nothing; Passchendaele.
However good an idea it may have been to begin with, cockup piled on misfortune piled on unlucky accident turned the battle into a slow motion horrorshow of shellfire and mud, mud, mud, where the only solid objects were the burnt out tanks and the bodies of the dead, and those only temporarily.
Second place, have to admit I don't know enough about the details, have to look them up, but China, 1860-1950, from the Taiping rebellion to the end of the civil war, was the scene of many battles of astonishing brutality and futility, that achieved nothing and advanced no goal except possibly population reduction.