Since their respective Marvel Comics debuts, fan debates have raged over which Avenger is stronger between Hulk and Thor, though what fans may not know is that Marvel revealed the truth of their power-gap decades ago.
Bruce Banner aka the Hulk became the mutated Avenger after he was exposed to an explosion of gamma radiation. Thor, on the other hand, is and has always been the God of Thunder (not counting when he originally had an alter ego, Donald Blake, who turned out to be something of an unwilling and vengeful avatar of Thor’s power). One hero was made, the other born, but both were immensely powerful. If one is to consider raw cosmic power alone based on their most impressive feats, then Thor seems to be stronger since Thor once murdered the seemingly unkillable Galactus. However, the Hulk did bring the world to its knees in World War Hulk and was able to defeat the One Below All with a single Thunderclap, so all-in-all it’s still a toss-up. Luckily, readers don’t have to think too hard on the topic because Marvel already gave fans the answer they’re looking for.
In Avengers #51 by Roy Thomas and John Buscema, the villainous Collector has taken over Thor’s mind and is forcing him to capture every member of the Avengers, past and present, in order to complete his sick collection. Having no say in the matter, Thor is ready to jump into action and go after his first target, but first the Collector needs to figure out who that first target should be. While scanning the galaxy for any and all traces of the Avengers, the Collector decides that Thor should attack the Avengers who are on Earth first, and then go after those who were on the outer reaches of the cosmos. One of the Avengers who was off-world was Hulk which meant he was momentarily off limits, but that didn’t seem to bother the Collector in the slightest. In fact, the Collector seemed relieved that the Hulk wasn’t going to be the first Avenger Thor had to take down, which is made clear in a comment the villain made to himself, saying, “Ah! The gargantuan green-skinned Hulk… he whose power is virtually the equal of even Thor’s!”.
One thing to keep in mind about the Collector is that he is an Elder of the Universe, meaning that he has been around since before the Big Bang and is a survivor of his previous universe along with the other Elders. Not only that, but as an Elder, the Collector has access to the Power Primordial which is the residual energy left over from the universe-creating explosion of the Big Bang. In other words, the Collector is very old and very powerful, so if someone was able to correctly power-scale other Marvel characters–who would appear as infants to him in age comparison–it would be him. With that in mind, the Collector said that Thor and Hulk are virtually equals when it comes to power, which essentially means that the Collector determined that the power-gap between Thor and Hulk is so miniscule that it can’t even be told apart by an Elder of the Universe.
Since the Collector sees Hulk and Thor as being virtually equals in terms of their respective strength, it is safe to say that the argument over which is stronger is completely pointless as Marvel Comics essentially confirmed they wield basically equal amounts of power–something that was confirmed by someone who would absolutely be able to tell which of the two is stronger since his billions of years of being alive would have offered him some form of power-gauging techniques, either technological or otherwise. So, in short, Hulk and Thor are so evenly matched that not even a being older than gods can tell the difference, which confirms that their power-gap–or virtually lack-there-of–has already been settled by Marvel Comics.