DC Comics’ pinnacle hero, Superman, has seen his fair share of fights from both cosmic and earthbound villains alike, but even he admits that none were as strong as Marvel Comics’ Hulk. The Hulk has proved time and again that he can take on pretty much anyone within the Marvel Universe and put up a decent fight - even Thanos admitted he was scared of fighting Hulk during the Thanos Quest storyline. When Superman encountered the Strongest Avenger, he understood Thanos’ fear and said the Hulk was stronger than anyone he'd faced before.
In The Incredible Hulk vs Superman by Roger Stern and Steve Rude, Superman and the Hulk are inexplicably in the same universe, with Superman wary of the green rage monster due to his destructive potential. When the Hulk loses control in a populated area, Superman flies to his location to stop him.
As the two begin fighting, Superman realizes that the Hulk is much stronger than he initially gave him credit for. At one point, Hulk even launches Superman into space, leaving Superman to marvel at the Avenger’s strength and speed exhibited during the battle. While in space, Superman says to himself that Hulk is, “Stronger than anyone I’ve ever gone up against” which is a huge feat given this comic’s publication history. The Incredible Hulk vs Superman was released in 1999. That means by this time, Superman had faced Shazam several times, and been (nearly) beaten to death by the monstrous villain Doomsday in Superman #75. In fact, Superman had faced pretty much every heavy-hitter in the DC Universe, friend and foe, showing just how mighty he considered Hulk to be.
While the Hulk impressed Superman in this issue, it wasn’t the first time he tangled with a DC Comics’ A-Lister. In the crossover one-shot Batman vs The Incredible Hulk by Len Wein and Jose Luis Garcia Lopez, the Hulk is fighting Batman after he is tricked by the Joker into doing so. While the Hulk may be the strongest fighter he has ever faced, Batman proved he isn’t the smartest as Hulk was easily fooled by the Joker into attacking Batman. Marvel and DC's crossovers have long been a chance for the two sets of characters to pay each other due respect, and Hulk's appearances have repeatedly stressed that while he may not be quite as strong as Superman, he does stand head and shoulders above every other hero and villain in terms of pure strength.
However, just because Superman is one of the strongest beings in all of comics history doesn't mean Hulk couldn't take him down in the right circumstances. After all, if Superman himself ranks Hulk above Doomsday, he's acknowledging that like DC's ultimate monster, Bruce Banner's alternate form has the strength needed to put the Man of Steel down for good. While fans are free to debate who'd actually win in a fair fight, Marvel die-hards will be happy to know that while Hulk is canonically weaker than Superman, even the Man of Steel finds his strength intimidating.