8. Hal Jordan's Fall From Grace
After the destruction of Coast City, Hal Jordan's former home, he was overwhelmed with grief. After a dispute with the Guardians, Hal is driven insane by a mixture of grief and frustration with the way the Green Lantern Corps is ran. This conflict eventually leads to him becoming the villain Parallax.
Hal's fall from grace was given multiple storylines to really explore the depths that he went to, all coming to a head in the crossover event Zero Hour: Crisis in Time.
The story largely serves as a follow-up of Crisis on Infinite Earths, but the constant throughout all of it is the heavy feeling the heroes feel while fighting their old friend - in particular Hal's best friend, Green Arrow, who is the one that ultimately has to stop him.
A devastating loss for both the League and for Oliver Queen, the story leaves behind a bitter taste, watching a core member of the Justice League and a staple of the DC universe fall so far.