When thinking of indestructible superheroes, Superman might be the first thought, Venom has one power that the Man of Steel just can't match. This power was revealed in the one-shot Venom: The End. The issue was written by Adam Warren and illustrated by Jeffrey "Chamba" Cruz. While Superman Vs. Venom is a popular matchup for fans, an actual showdown wouldn't be close.
Venom's The End story takes place in the far future of the Marvel universe. After all life throughout the universe has died, the symbiotes remain along with an army of techno-organisms that seek to make a no-organic zone across the stars. Venom hopes to unite the other symbiotes against them. Unfortunately, the symbiotes are better at fighting than they are coming together as a team. A symbiote war raged in which Venom was the only survivor.
Forced to fight the machines by himself, Venom searched within his codex of previous hosts and found the X-Man Jamie Madrox, who had the ability to multiply himself. Through this power, Venom created an army of himself to fight. But that still wasn't enough. To unleash the ultimate weapon, Venom found a way to extend himself through space-time, which resulted in all matter that the Venom matrix occupied being disintegrated. So essentially, Venom destroyed existence in order to wipe out the machines.
Superman can withstand a lot, but he is still made up of matter. If, in this far-flung future of the Marvel universe, Venom's ultimate form ever encountered Superman, he would just be able to vaporize him. Also, the Man of Steel is small in stature compared to the machine army Venom had to face in this war. It would take up far less space for Venom to inhibit in order to dissolve Superman's atoms. Unfortunately for the Man of Steel, Venom wouldn't even have to go that far to subdue the Kryptonian hero. The Marvel and DC crossover comic DC/Marvel: All-Access #1 sees Superman actually facing Venom... and being defeated by Venom's black webbing. Still though, that victory pales in comparison to what Venom's ultimate form could do.
The amount of space Venom would have to occupy is an important detail. In the issue, it describes how this power is incredibly painful for Venom. The narration describes it as the equivalent to "a human ripping out their own intestines to strangle an enemy...then stuffing the organs back inside themselves." It then goes on to say it's actually much more painful than that. Still, the fact Venom can do this is immensely impressive, and Superman wouldn't stand a chance.