It's a commonly held idea that Superman is the most powerful superhero in the known comic book universe. There's a reason he's frequently been painted with messianic elements, and why every time he dies or is even vaguely defeated there's a massive fanfare. Not to put too fine a point on it, but it's because God doesn't often get his ass handed to him.
But look a little closer at the ranks of superheroes who have appeared on the pages of comics over the years and you'll find that there are more than a few pretenders to Superman's title. Sure, he has super-speed, super-strength, heat-vision and any number of ad-hoc powers that have been introduced in his film universe to serve the plot, but he set a benchmark for superheroics and others have come in well above it since. He's not quite as infallible or as perfect as once thought.
In a toe-to-toe battle, there are actually a number of heroic characters who could knock seven bells out of Earth's mightiest protector, and that's the easiest metric for working out whether they're more powerful. So much for being World's Finest...
Confusingly once named Captain Marvel, Shazam is another superhero who can pretty much directly rival Superman in terms of most of his powers, and comes with an added kicker that the Man Of Steel cannot contend with. Again, it's magic that would be his undoing.
Shazam's powers are also handily easy to define, he has the wisdom of Solomon, the strength of Hercules, stamina of Atlas, power of Zeus, courage of Achilles and speed of Mercury (spelling out his name, WHAT A COINCIDENCE!). So in other words, he's Superman, only powered by the Gods and with the ability to use magic in battle.
And crucially, Superman has actually conceded that he couldn't beat Shazam in the past...