1. Doomsday: In the iconic "Death of Superman" storyline from 1992, Superman faced his demise after an intense battle with Doomsday, a creature created using genetic engineering as the "Ultimate Weapon." Doomsday's immense strength and adaptability made him a formidable opponent for Superman, and the battle resulted in their mutual defeat with Superman sacrificing himself to save Metropolis.
2. Darkseid: In some storylines, Superman has also faced threats from Darkseid, the ruler of the planet Apokolips. Darkseid possesses powerful Omega Beams that can potentially harm Superman, and in some instances, he has been able to defeat Superman through manipulation, cosmic power, and superior strength.
3. Imperiex: Imperiex is a near-omnipotent being known as a "Super Celestial," and during the "Our Worlds at War" storyline, he threatened the existence of Earth and the entire universe. Superman and other heroes confronted Imperiex and sacrificed themselves to stop his destructive force.
4. Kryptonite: Kryptonite is a radioactive mineral that originates from Superman's home planet, Krypton. Prolonged exposure to Kryptonite can weaken Superman's powers, making him vulnerable to attacks. Lex Luthor, one of Superman's notable adversaries, has frequently exploited Kryptonite's effects to try and defeat Superman.
5. Red Sun Radiation: In some stories, Superman's powers are diminished under a red star's radiation, similar to the sun of his home planet Krypton. This weakness can potentially make him vulnerable if he's exposed to intense red star radiation for an extended period.
It's important to note that in most cases where Superman is killed, he often returns. DC Comics has a long history of employing retcons, alternate timelines, and resurrections to bring back its iconic characters, ensuring that Superman continues to be a central figure in their comic book universe.