The strongest superhero is going to require one of the deadliest rogues galleries ever assembled. The Man Of Steel has villains in spades, and there are quite a few of them that give him a run for his money on a consistent basis. To set the parameters for this list, these villains aren't ranked in terms of physical strength; instead, they're ranked in terms of how strong they are as an adversary to Superman. This means brains, brawn, and/or something else up their sleeve (or robotic arm). Secondly, each villain on this list is primarily a foe of Superman; not someone Superman has occasionally fought, and not someone who is regularly a villain that requires the work of the entire Justice League.
10 MISTER MXYZPTLK
A villain with a name that will tangle even the most super of tongues, Mister Mxyzptlk (the pronunciation has changed over the years) is a being from the 5th dimension. Because of the fact he hails from the 5th dimension, he has certain abilities that appear to be magic to our eyes. Originally, Mxy acted simply as a prankster, but in more recent years, his antics have gotten a bit darker. His actions have become more and more dangerous, and the only way to stop him (depending on the story) is to either trick him into saying his name backward or into another parameter that Mxy himself sets.
The story of Ultraman has drastically changed over the years, but one thing has remained constant: the threat that he poses. Ultraman is an alternate universe version of Superman, usually Earth-3. In this universe, most of the heroes we've come to know and love from our universe are evil, and many villains we know and love to hate are good. Ultraman, instead of gaining power from a yellow sun, is weakened by it. However, Kryptonite (specifically the green variety) either strengthens his powers and/or gives him entirely new ones.
How does the strongest man alive fight a villain that he can't hit? Therein lies the rub with Parasite, one of Superman's most difficult foes. Parasite gathers energy from just about any source of it, including kinetic energy from anything coming into contact with him. If Superman were to level Parasite with a punch, it would only make Parasite stronger. In certain iterations, once Parasite comes into contact with somebody, he also gains some of their memories. This became a significant problem when Parasite came into contact with Lois Lane and learned of Superman's identity.
7 CYBORG SUPERMAN
Originally masquerading as the real Superman following Superman's apparent death at the hands of Doomsday, Cyborg Superman managed to fool many into believing he was the real thing. In reality, Cyborg Superman is really Hank Henshaw, an astronaut on a ship that was hit by a solar flare.
The radiation from the event eventually killed Henshaw and his entire crew, but Henshaw managed to save his consciousness with some kind of technology. Because of the nature of the incident and Superman's action that may have caused the solar flare, Henshaw blames Superman for his condition.
John Corben was a small-time con who was nearly killed in a car accident. Scientist Emmet Vale saved his life, putting Corben's brain in a robotic body powered by Kryptonite. Vale instructed Corben to kill Superman, but Corben instead killed Vale and ignored the command. Corben, now known as Metallo, used his new body to commit more crimes, eventually coming into contact with Superman. Because of Metallo's unique energy source, he poses a significant threat to Superman and has been a consistent antagonist for him for decades.
Bizarro is a true hero, unlike the villain Superman. Or so, he thinks he is. Generally, Bizarro is a botched clone of Superman, and though not always evil-natured, he's the opposite of Superman in most ways, resulting in him perpetrating many destructive acts in the name of being good. Or, well, bad. Or- you know what, forget it.
Because of Bizarro being the opposite of Superman, some of his powers are different, such as his freeze vision and fire breath.
Most versions of Brainiac depict him as a being seeking knowledge, to the extent that he's willing to use any destructive force necessary to obtain it. This resulted in the shrinking and bottling of the Kryptonian city of Kandor, an act that Superman was unable to reverse.
Brainiac uses his immense intellect to challenge Superman regularly, at one point going so far as to construct an entire artificial planet in an attempt to defeat him.
3 GENERAL ZOD
General Zod is usually depicted as the military leader of Krypton, who survived the planet's destruction and is hellbent on the destruction of Earth, sometimes for the purpose of rebuilding Krypton, and sometimes just because he's a bad guy. Because he's a Kryptonian, he has more or less the same powers as Superman, which presents a huge threat not only to Superman, but to the rest of the world.
The supposed end of Superman came at the hands of the monster known as Doomsday. Generally, Doomsday has been depicted as a creature designed by someone on Krypton and created only for hate and destruction. His long journey resulted in a lot of death and destruction before he finally made landfall on Earth, hitting the ground so hard he was buried. Eventually, Doomsday beat his way out of the ground and began sowing destruction across everything he saw, leading him to Superman. Superman and Doomsday had a violent fight that lasted a long time, eventually ending in Metropolis with what was thought to be the death of them both.
1 LEX LUTHOR
After all of these villains either born with abilities, gifted with powers, or created for destruction, it ends with... an ordinary man (though he would scoff at the suggestion). Superman has so many other foes that rival his strength and seem destined to come to blows with him, but his greatest adversary of all time is Lex Luthor, a man who hates Superman because, in his mind, Superman is a threat to humanity. Lex Luthor cannot come to terms with the fact that humanity nigh-on worships an alien being, almost as if he's a god. To sum up Luthor's motivation perfectly, a quote from Batman v Superman:
"If God is all-powerful, he cannot be all good. And if he is all good, he cannot be all-powerful. And neither can you be."