The deaths of Sue Dibny and Jack Drake, especially with the aftermath that follows.
"Batman and Robin. Orphans."
Watching Ralph fall apart at Sue's funeral is heartbreaking, especially since you've been reading several pages of how much he loves her before the murder.
Even worse, when he actually finds her and literally has trouble holding himself together.
Following the funeral, he similarly can barely hold himself together, literally having to support his chin with his hand as he sobs uncontrollably. Further compounded by the look of absolute grief on his face
Batman's scene at his parents' grave
Mom, Dad. Watch over Jack Drake... and Sue.
Atom shrinking down to nothingness out of grief.
I never felt so small in my whole— *
The look on Firestorm's face, knowing he's about to die.
Firestorm: T-the professor... my family... someone say goodbye to my Dad for me... (flies away)