Long-time fans of Superman know that the Man of Steel has been said to be "more powerful than a locomotive." While this may have been an apt description of the Golden Age Superman, Kal-El has long since surpassed that meager limitation - to the point where it's virtually impossible to find something comparable to Superman's strength level, as the Man of Steel has shown time and again that he truly is the most powerful superhero ever.
Born on Krypton, Kal-El was already gifted with muscles far denser and stronger than any human thanks to his home planet’s heavier gravity and harsher atmosphere. His power was further enhanced when his parents sent him to Earth, where the rays of the yellow sun strengthened his muscles even more and provided him with a whole host of other powers, including flight, super senses, invulnerability, enhanced speed, and heat vision. But the true limits of his strength aren't as clear as even his greatest fans may realize.
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Since Clark Kent’s power source is the sun, it’s important to stress that Superman’s strength is continuously increasing the longer he’s exposed to its rays. Thus, while some modern versions of his origin state that he had human level strength and durability as a child, as a teenager and adult, Clark’s muscle power has grown exponentially year after year. So, how strong is Superman? Well, after roughly twenty or thirty years on Earth, Clark’s muscles have grown so strong he’s been known to pull continents apart or even hold black holes in his hands. At one point in Superman #13, Superman decided to work out with some advanced machines and found he could bench-press the weight of the entire Earth (roughly 5.972 sextillion metric tons) for five days straight.
When he gets really supercharged by the sun, Superman’s strength levels are virtually limitless. Once, in All Star Superman, he entered the sun to save a team of scientists and ended up able to lift 200 quintillion tons… with one hand. To top it off, Superman stated he felt like he could do more – indicating this wasn’t even close to his maximum potential. During the Silver Age, Superman actually manipulated entire suns like they were playthings – moving them closer to certain planets that had lost their previous sun, or even creating entirely new ones by smashing moons together! He also moved an entire galaxy to a sector of space with new suns… while he was still Superboy.
One interesting thing about Clark’s strength is that it isn’t completely dependent on muscle power. While Superman’s muscles are incredibly strong, the Man of Steel has noted on multiple occasions that objects feel considerably lighter to him when he’s lifting them while he’s flying. This indicates that when Superman lifts an object, he isn’t just exerting force with his arms and legs – instead he’s extending an energy field that helps him support all that weight. This would actually explain how Superman has been able to lift buildings into the sky without having them break apart – his energy is somehow holding the object together.
More recently, Clark’s revealed a new power called the “Solar Flare” which allows him to expel all the solar energy from his body at once, essentially turning him into a living bomb. The caveat is that this act depowers him for twenty-four hours until his body absorbs enough solar energy to re-energize his powers. While Clark is basically at human levels of strength at this stage, it’s remarkable how quickly he can recover his god-like strength when certain stories have shown he needed to absorb solar power for years before attaining incredible super strength. Clark can also further enhance his strength by absorbing the rays of a blue sun. Although the full effects of this type of solar radiation on Kryptonians is unknown, Clark found his normal powers were dramatically increased under a blue sun – and that he also got a special new power that let him give Kryptonian powers to normal humans.
As outlandish and, frankly, ridiculous as some of these strength feats might sound, it's important to note that Superman occupies a very unique place in the pantheon of modern day superheroes. Where characters like Batman or Spider-Man emerged to give humans someone to aspire to, Superman exists largely to give other superheroes someone to aspire to in terms of his strength and morality. As such, Superman needs to be essentially limitless in terms of power. He may be a god that would prefer to be a man, but no matter how many times comic book creators may depower Superman in hopes of making him more "relatable," sooner or later, the Man of Steel will return to his natural, limitless state.