Fans love to argue about who could beat up whomever and whatnot online, but these arguments are usually based on the arguer's emotional connection to the character and some stuff they've read on Reddit.
But if you want to present an airtight argument you've gotta have scientific evidence to back up your point, especially when you're arguing about superheroes. Which is why I trust quantum physicist Dr. Spiros Michalakis when he said Ant-Man can beat up Superman during an interview with Inverse.
Dr. Michalakis served as a consultant for the Marvel movie Ant-Man, helping the writers infuse some real physics into Ant-Man's abilities, especially his shrinking power:
“When I was looking to inject elements of modern physics into the script, I brought up this idea that, when Ant-Man goes into the Microverse and something malfunctions, he doesn’t just go to just a smaller space like Fantastic Voyage, (he goes to a step beyond the Proteus) … This is a place where the nature of reality changes around you. So, when you enter the quantum realm, its different set of laws takes hold.”
As Spiros sees it Ant-Man would be able to best Superman by sucking him into a black hole:
“What I’m saying is that potentially understanding the quantum code from which curvature of space-time comes from, [Ant-Man] could manipulate to increase it or decrease it.” Superman has, in the canon of the DC Universe, lifted 200 quintillion tons. But if Ant-Man had a way, he could crush Superman with 201 tons... Ant-Man could have created say, a black hole. Could Superman escape the black hole? Probably not. Then game over.”