For a hero like Superman, it's hard for some writers to give him any sort of a challenge thanks to his great strength. It's an unpopular opinion about Superman to say that he's best when he is vulnerable, and most well-written Superman stories show that the real threat to Kal-El isn't those who can match or surpass his strength.
Instead, the biggest threat to the hero has always been foes who have managed to outsmart him at every turn. What they may lack in terms of raw strength, they more than make up for with their intelligence, which they use to defeat the Man of Tomorrow.
Some targets may see the loud-mouthed, cuss-throwing, cigar-chomping, gun-toting, boisterous bounty hunter and imagine him to be a bit on the dimmer side. However, that's a mistake that they will only make once as they find themselves inexplicably cornered and at his mercy.
It's often forgotten by characters in-universe and readers alike that Lobo is actually intelligent. His brain is wired completely differently, allowing him to process events and knowledge incredibly fast. Whether it's creating massive guns from actual junk or tagging the likes of The Flash by quickly calculating where he'll end up, Lobo's smarts are just as lethal as his strengths. Plus, Lobo knows he's in a comic book, as the DC fan Leslie unfortunately found out.
Toyman is one of Superman's most iconic foes, especially in the animated DC universes. A toymaker who hides his psychopathic tendencies by running a toy store, his characterization has changed massively over the years. He's gone from goofy mad scientist to terrifying child serial killer, and then to serial child protector.
What has remained consistent across all personalities is Toyman's penchant for advanced robotics and weaponry. It takes a special kind of skill to be fighting Superman of all people as a normal human being and manage to annoy him another day. Toyman is the epitome of never judging a book by its cover because the book might just explode on the unsuspecting reader.
Hank Henshaw was exposed to cosmic rays that gave him superpowers, but at the cost of his physical body. Soon, he used his newfound powers and natural scientific intellect to create a cybernetic body for himself. The cyborg body was composed of Kryptonian parts from the very same pod that Superman arrived as well as some of his DNA.
Cyborg Superman managed to hide his sinister agenda during his time acting as "the new Superman" during the infamous stretch of time when Superman was considered "dead" after his fight with Doomsday. As soon as Superman came back, however, he revealed his true colors, and it takes all of Superman's strength to put him down. Well, for a time at least. He has since become one of the foremost leaders of the Sinestro Corps, and even acted as the Herald of the Anti-Monitor, proving his smarts throughout.
Jax-Ur is Krypton's resident mad scientist, who often creates all those doomsday devices that General Zod loves to threaten planets with so much. His entire character is based on the famed Nazi war criminal "scientist" Josef Mengele. Much like that terror, his experiments were often thinly-veiled ways to sate his sadistic curiosity.
His experiments were of such reckless scientific abandon that they led to the destruction of an entire moon colony and inevitably led to the loss of Kandor. Jax-Ur is always sure to be scheming a plan under any Kryptonian leader he follows, and they should always watch their backs.
The Ultra-Humanite, in addition to having an awesome name, is also an awesomely intelligent villain. Ultra-Humanite also has the honor of being Superman's first-ever archnemesis, even before the likes of Lex Luthor. Once a mad scientist intent on taking down Superman, he has since developed the ability to surgically transfer his brain into other vessels.
Ultra-Humanite is a wickedly cultured scientist whose goals seem to be more about progressing humanity in his own mad way. Although his comic appearances are villainous, portrayals in media such as Justice League Unlimited show a much softer side that truly values intelligence as a means for betterment.
General Zod is the first taste comic book readers get as to how horrible it would have been for Earth had Superman decided to be a conqueror. As a Kryptonian of high ranking in both the military and in politics, General Zod is a genius strategist and tactician, often leading from the front in his battles.
Not to mention, he is also an incredible diplomat. All of this intelligence is thanks to being raised by two brilliant Kryptonian scientists who instilled within Zod the values that he would hold dearly. Zod isn't just a threat to Superman because they have the same powers. He's a threat because he has the intelligence to know exactly how to use those powers to hurt Superman and his beloved planet Earth.
Mongul might be considered by many to be a "poor man's Darkseid," but if one stops to consider what that means, they better start running. Mongul might seem like Darkseid at a glance, but his methods are far more "hands-on"' so to speak, and he is more likely than the Lord of Apokolips to get down and dirty in his scheming.
Mongul has used his strength and intelligence to conquer countless numbers of worlds, and he does with terrifying casualties. His destruction of Coast City alongside Cyber-Superman should be proof enough of his cunning. Mongul often makes use of the fact that some people view him as a brute to craft devious plans. which more often than not, end with those who underestimated him kneeling beneath his feet.
The fallen new god Darkseid is the most threatening being in the entirety of the DC Universe. His power is near without equal, and numerous Earths in many realities have fallen under his bootheel. While some people might attribute his success to his sheer power, his intelligence is the true secret to his success as the harbinger of Anti-Life.
Darkseid is the antithesis of everything that the new gods stand for. His quest is to dominate all life in the universe, and eventually, crush it all in his fist. Darkseid has outwitted gods and stood toe to toe with the highest level intellectuals in the galaxy. However, he does have his limits, and usually delegates significantly advanced technology to his legions of enslaved scientists.
Whether he's a green alien with advanced technology or a Kryptonian supercomputer intent on putting all life into stasis for his "research," Brainiac has proven to be one of Superman's most effective foes, and all purely through the power of his mind. His true form has him equal Superman time and time again, and it's alleged that the destruction of Krypton can be attributed to the intellectual's endless pursuit of knowledge.
Brainiac's single-minded pursuit of knowledge is his greatest strength, but it's also been the source of his greatest weakness. His narcissism and complete inability to adapt to things that fall outside of his plans have cost him victory against Superman more times than he cares to admit. Still, Brainiac is de-facto the smartest being in terms of raw knowledge and processing power. However, there is one who might be a "smarter" being all around.
Lex Luthor prides himself on being the opposite of Superman in every way. In his mind, he is a truly self-made man who has earned his physique and his intelligence not from genetics or the sun. Instead, Lex Luthor gained everything he has because he worked for it. With blood and sweat, he built his empire on Earth, and to him, Superman was trespassing.
While he may not have the godlike powers of Darkseid and the innate processing power and knowledge of alien beings like Brainiac, Luthor has managed to outwith both of them time and time again. In Luthor's best film and TV portrayals, his humanity is the key to him thinking in ways that no god or computer would ever stoop to think, and that gives him the edge over these two insanely powerful beings.
