Doomsday famously killed Superman and is generally reckoned to be the only true match for Superman in a one-on-one fight.
But are there any other villains who've killed Superman in the comic serials?
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Superman was killed by Murder Man. And yes, that's his name. I'm not joking.
In Superman #188 Zunial (AKA Murder Man) turned a kryptonite radio transmitter on Superman and killed him. Luckily he wasn't 'Uncle Ben dead' and one of his Superman robots was able to restore him to life.
Superman was killed by Lex Luthor in the 'What if...' story called "The Death of Superman!". Tricky Luthor invented a cure for cancer that happened to be based on green Kryptonite. Once the world was saturated in the rays, he turned the beam full-strength on a highly weakened Superman, killing him instantly.
Superman was killed by He-Man in the crossover event DC Universe vs. The Masters of the Universe #02. It won't surprise you in the least that they found a way to bring him back two episodes later
The Superman that died was from another reality
In the one-shot "The Kingdom", Superman is killed over and over again (in a variety of exciting ways) by Gog.