The identity of Arguile after his death? Hiraga Saito. Canon is now completely destroyed.
Rip Jack's final moments. Despite everything that he has done, you can't help but feel at least a little bit sorry for the poor delusional fool.
Chapter 12, part 3: Yui breaking apart like a porcelain doll, asking Kirito to forgive her for being reckless again.
In the Beach Episode chapter of 3.0 Yuuki confides in Shiune about her Survivor Guilt for having outlived her sister just long enough to get a new body and lease on life thanks to the Transition.
The Beach Episode again. Poor Djinn-kun. Sometimes it's the small losses that hit the hardest.
Montmorency delivering news to Ianith. The worst part? Her husband had probably never really existed other than as an entry in a quest log.
Louise and Kriche in the hospital after Louise casting faerie magic. Their entire friendship is all but destroyed. And then Koko heard them talking...