Superman took down the X-Men's most unstoppable enemy, the Juggernaut - with one punch. The stepbrother of Charles Xavier, Cain Marko stumbled across the legendary Crystal of Cyttorak and was transformed into a human Juggernaut. Since then, he's gone toe-to-toe with some of Marvel's most powerful superheroes, matching the likes of the Thing and the Hulk.
"Nothing can stop the Juggernaut," Cain Marko likes to yell, and it's more than just a battle cry. When the Juggernaut starts to move in a given direction, he's literally unstoppable. Some Marvel fact-files have suggested he generates an impenetrable force-field that means nothing can arrest his momentum. There's something phenomenally intimidating about this powerhouse, who can press on regardless, his progress unimpeded by anything that's in his way. Even the Hulk has been forced to resort to drastic measures to stop Juggernaut - he once famously simply picked up the rock the Juggernaut was standing on and tossed it into space.
But Juggernaut met his match in DC Versus Marvel #1, in which he was spontaneously transported from one universe to another. He jumped from a scrap with the X-Men Metropolis, and his fist cracked the wall of the Daily Planet. A bewildered Juggernaut barely had the time to wonder where he was before he was knocked down by Superman - with just one punch. "I don't know who you are or where you came from," Superman observed, "but I do know you're in the wrong place, mister!"
This should easily go down as one of Superman's greatest feats, as he unknowingly demonstrated greater strength than almost any Marvel superhero. This is all the more remarkable when you consider that Superman was probably pulling his punches; he had no idea who he was taking on, and could easily have hit with too much force. Juggernaut was left cowering on the ground, visibly stunned at the sheer force of the blow he'd just received.
Of course, it's interesting to speculate whether Juggernaut was still operating at maximum strength. His powers are derived from another dimension known as the Crystal Cosmos, and there's no reason to assume the energies of that dimension could cross into the DC universe with ease. Still, at this point Juggernaut had barely made his omniversal jump, so he was presumably still packing all the power he had before. Given that's the case, it comes as no surprise that, later in the DC Versus Marvel crossover, Superman dealt with the Hulk as well. He'd already unknowingly proved himself to be far more formidable than one of the Hulk's most challenging foes, and so the outcome of that particular versus match was pretty easy to predict.