What OP character's death do you regret the most?

Now that a certain character is going to die in the next chpt or so, I find myself reflecting on past OP character deaths that have hurt me. On this feeling of bitter regret that certain characters aren't still alive. I feel bad for Ace who was told by the world he shouldn't exist, but mostly I feel so bitter about Fisher Tiger who could've lived if he was able to let go of his hate.

Anyway, what character deaths have you wished never happened?