4. The Parasite
The Parasite is a bizarre creature, a man mutated by radiation to develop life-draining powers and the ability to leech the powers of whoever he lays his hands on- crucially, he offers a writer the chance to depower Superman without resorting to the overused Kryptonite. There are a great number of scenarios that could be very interesting to see played out between the Man of Steel and the Parasite: imagine a powerless Clark Kent coming to terms with his own vulnerability, forced to use his wits alone to bring down a creature with all the powers he used to have.
Though the character has stood the test of time, he has also been chronically underused, and will be far less recognisable to an audience than even Doomsday. Fans of the DCAU may recall his appearance in Superman: The Animated Series, but his personality was never well-developed. In addition, a version of the character appeared on Smallville, but aside from stealing Clark’s powers he bore little resemblance to his comic counterpart. A reinvention is in order, as with so many overshadowed comic-book villains, and I believe a film could do the job brilliantly: the Parasite is dangerous and unnerving, and his concept seems to cry out for a film writer to make good use of him.
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.