SUPERMAN 4: Brainiac, the Eradicator.
The Eradicator starts as a villain in service to Brainiac, but he switches swides and joins Superman, and together they defeat Brainiac.
SUPERMAN 5: General Zod, Ursa, and Non.
The Kryptonian criminals are defeated through clever trickery by Superman, only to be approached afterwards by Brainiac, who suggests an alliance. They use Kryptonian technology to transform Non into Doomsday.
SUPERMAN 6: Brainiac, General Zod, Ursa, Doomsday.
Unable to beat them on his own, Superman gets the help of the Eradicator, Metallo, Parasite, and Lex Luthor. The Eradicator, Metallo, and Parasite take down Brainiac and Ursa while Luthor kills General Zod. Luthor finally gets his moment in the sun, his chance to be redeemed and be acknowledged as Metropolis's greatest hero by killing General Zod...
...and then Superman dies a martyr in battle against Doomsday, robbing him of his moment.