3. General Sam Lane
The greatest strength of Lex Luthor as a character is that he is human. He is everything Superman is not, forced to rely on his wits and his scheming to bring him down, and he is a constant reminder that not everybody in Superman’s adopted world want him here. General Sam Lane, the father of Superman’s perennial love interest Lois Lane, is also all of the above. A military man and initially presumed dead, the General is a villain who pulls the strings from behind the scenes to undo the Man of Steel, believing the alien superhero represents a threat to the human race. Where Luthor often operates outside the law (putting aside his stint as US President), Sam Lane acts with the support of the military establishment and shrouded in secrecy. The law is not his enemy, as it is Luthor’s; it is his tool.
He is, in many ways, DC’s equivalent to General “Thunderbolt” Ross, the military enemy of the Hulk in Marvel comics, recently portrayed in 2008’s The Incredible Hulk by William Hurt. Many of the most insightful superhero works have messed around with the traditional understanding of the “Superhero Vs Supervillain” recipe, and Superman’s opponent need not necessarily be a traditional “supervillain”. Case in point…
More info on Doomsday and Brainiac you left out. Doomsday, in another comic, cerca 1994, his origins are revealed. He was an experiment in genetic modification on Krypton. That’s why he hates Superman, because he was tortured and experimented on Krypton. Also, he is currently trapped on the moon, being constantly divided between four teleporters, making it so that he can never be more than 25% complete at any one time. Finally, Brainiac once took over Doomsday’s body, providing the most formidable foe ever fought by Superman. However, Brainiac wanted a more refined body and that’s how he was beaten.
Finally, I would make an addition to this list: Hank Henshaw. Don’t know him? Look him up.