d: The way I remember it, Superman said he could not be sure that he did not kill- whatever that “Mxyzptlk” thing was- on purpose, at least subconsciously, and as long as there was a doubt, he no longer deserved to be Superman. (Yeah, that was a load of BS, but so was the rest of the story.) However I COULD be wrong, I only read the story once, and that was over 20 years ago, nor do I feel like ever reading it again, so I guess we’ll just have to leave it there.
(And once again: I do NOT mind corrections or additional information, in fact I welcome them!
)
OK, now let’s move on to the next Bald Superman Villain (yeesh, just join the Hair Club For Men already, you guys!): Brainiac!
Of all of Superman’s villains, Brainiac has to be the one who has undergone the most radical changes in appearance and nature. As originally introduced, Brainiac was a space criminal, a green-skinned alien who had artificially given himself “Twelfth-Level Intelligence!” (Compared to humans’ lowly level-six intelligence… according to him, anyway.)

(Whoa, that was a cool introduction!) 
Brainiac used his power to invent quite some amazing technology, the most outstanding being his force field (that not even Superman could break) and a shrinking ray powerful enough to shrink entire cities down to the size of scale models! Brainiac in fact did go around stealing cities from various planets, then charging them ransom for returning them. This included Kandor, a city from Krypton, though the planet exploded soon after the theft, so Brainiac just kept the city- in a bottle-shaped device that simulated Krypton’s environment- as a souvenir. After his first encounter with Brainiac, Superman recovered Kandor but the shrinking machine had run out of power, so there was no way to return the now-microscopic Kandorians to their normal size. So Kandor ended up as another cool display in Superman’s fortress, though he did promise them that he would find a way to restore them to normal, which he did- but not until the early 1980’s.
Brainiac became a very popular villain. So much so, that when the comics introduced the 30th Century’s Legion of Superheroes, one of them was Brainiac 5, a descendant of the original villain! While B-5 had inherited his great-great-grandfather’s intelligence, he was determined to do good with it, to clear his family’s name.
…However, in yet another sign that DC Comics never patrolled their continuity particularly well, a later Superman story established that Brainiac was actually an android! This of course meant that he could not be Brainiac 5’s ancestor. They had to explain it later by saying that Brainiac had been created by the Computer Tyrants of the planet Colu in the form of a Coluan to infiltrate them, and he in turn adopted a Coluan child to increase his disguise. It was this child who was Brainiac 5’s true ancestor.
It should be noted that, in popular culture, the name “Brainiac” has been used as a pejorative on intelligent people, at least in the past. It is not clear if this was because of this character. Another story claims the character was named (as were many others) after the Univac, and early type of computers Or perhaps it was after Multivac, a computer in Isaac Asimov’s science fiction stories. There was also a Mexican horror movie in the 60’s called The Brainiac, about a brain-eating alien, but that was the title given to the English dubbed version (it was titled “The Baron of Terror” in Spanish.)
Brainiac became one of Superman’s best known villains, teaming up with Lex Luthor in several occasions, and being a regular member of the Legion of Doom (in the Super Friends cartoons.)
Still, by the 1980s, DC comics felt that the character had to be reinvented to be made more sinister, as they had done with Luthor. In fact, it was in the same comic that Luthor got his power armor that the change took place (but in a separate story.)
Basically, Brainiac, who had been trapped inside his own Death Star-like space base by Superman, tried to escape by teleportation, but something went wrong. He was instead turned into a cloud of sentient particles! These particles traveled trough time, all the way to the beginning of time, where they encountered “The Hand That Created The Universe”, and it tried to destroy Brainiac, but it escaped back to its base, where it had its machines recreate it into a new form- looking sort of like an robotic skeleton:

Oh, and it was now supposed to be a “techno-organic” being rather than just a robot. Ironically, he acted less human in this form, and its main directive was now to… find the means to destroy He Who Would Destroy Him. (Not Superman. God. Yes, GOD. Personally, I think Brainy just went mad and imagined the whole time travel thing, but it was never explained either way.) Anyway, its plans always ended up endangering Superman and others, so it was still a menace. I found this new version quite pathetic, but at least it had a pretty cool spaceship:

Some of you may have seen the “skeleton” Brainiac in some of the later Super Friends episodes (AKA The Super Powers Show) though no explanation about its change of forms was given on TV, that I remember.
Thankfully, “Skeletoniac” didn’t last long. The big DC Reboot of 1985/86 had Brainiac return as…
…A stage magician?
Yup. Apparently, as a way of explaining his silly name, John Byrne (the writer/artist in charge of reinventing the Superman mythos) introduced us to Milton Fine, a minor psychic who was known as “El Brainiac!” and who claimed an evil presence from the planet Colu was trying to possess him. It turned out that he DID possess enormous psionic powers and, under the alien’s control, went in a rampage that Superman stopped. Though at first it wasn’t clear if he was right or if his mutant powers had driven him mad. (As you can guess, he WAS saying the truth; the real Brainiac was a Coluan named Vril Dox, who betrayed his people to serve the Computer Tyrants, and was rewarded by being disintegrated. But, like the previous version, it continued to exist ethereally, and was indeed trying to use Milton’s body and powers. Ironically, a brain operation to remove a tumor (which was blamed as the cause for Milton’s “hallucinations”) instead effectively wiped out his personality, leaving the alien Brainiac in control. Poor fellow.)

Afterwards, Dox used his technology to change Fine’s body into a version of his original one (though he kept Milton’s goatee.) He also kept the name out of cruel irony. Oh, and he made a spaceship identical to the “skeletron” Brainiac’s; he claimed the idea “came to him in a dream.”

(Funny note: According to some Hollywood rumors, this version of Brainiac was going to be used on the fifth Superman movie- played by Tim “The Tool Man” Allen! However, that movie was never made.)
Brainiac’s next mayor reinvention came with the Animated Superman series in the 90’s. For some reason, not only they used the Brainiac-as-an-alien-robot idea again, but they made it Krypton’s main computer, and had it decide to abandon the planet to its doom. This Brainiac had the “pure logic but with universal destruction goals” personality of the skeleton Brainiac, but looked like the original. As fans of the series know, this Brainiac not only joined forces with Luthor but physically merged with him (as seen in a long-running plotline in the later Justice League cartoon). Luthor actually enjoyed the merger (as it gave him access to the stolen knowledge of entire worlds) and his obsession with finding and rejoining with Brainiac was his final motivation in the League series. Too bad for him, Darkseid joined with Brainiac first. But Luthor helped the heroes defeat the Darkseid/Brainiac combo in the series’ finale.

Currently, the comics have finally decided to return to a robotic version of the character, very similar to the animated one in that it’s an ancient computer that steals knowledge from entire worlds; however, they claim that it has been creating “lesser versions of itself” for centuries, and that the Brainiac who possessed Fine was one of them. (Where does that leave Brainiac 5 again? I don’t know. The Legion ALWAYS gets screwed whenever DC wants to retcon its continuity.)
Anyway, Brainiac is definitely among the best known Superman villains (mostly from his Legion of Doom days) and is among my favorites, as well. However, I feel that his constant reinventions probably only serve to confuse the fans about exactly who Brainiac is supposed to be: A robot? An alien? A technorganic being? A telepath? A planet-eating computer? Stick to one version, already!
Next: Finally, a Superman Villain with hair! Even if it’s all messed up: BIZARRO!