For decades Marvel danced around the question of which hero is the most powerful? They did create a hierarchy with a select few at the top. This was refined by several notable battles. The first was the original Hulk vs the Thing fight. This was an important issue. It wrapped up the Hulk's run on his own comic and set up for him to move into the back pages of Tales to Astonish. It also had the Avengers. Things like that just didn't happen in comics in those days. Regardless, the main even was the Thing trying to stop the Hulk even though the Hulk was stronger.
Not long after an issue of Thor featured a Thor/Hulk fight. This was a bit of a cheat. It was done as a flashback of a fight between the two in Avengers 3. Through a special dispensation from Odin, Thor's hammer was unenchanted for a few minutes so that they could meet hand-to-hand. The fight was still inconclusive when the time expired and Thor recovered his hammer.
The two met a few times afterward with no conclusion. In an Avengers/Defenders crossover, the two strained against each other without giving an inch for hours until the rest of the heroes told them to knock it off. This obviously violated the Hulk's most basic attribute - the madder Hulk gets the stronger he gets.
For a short time the Hulk and Thor were at the top of Marvel's pyramid. Then new characters were introduced. Thor fought Hercules a few times with results that were just as inconclusive. Hyperion, a Superman surrogate, moved into the Marvel universe from an alternate world (although Thor used his magic hammer to beat him when they first met).
During the Silver Surfer's short run in double-sized comics, he fought Thor. He won but concluded that Thor was stronger so Loki must have augmented the Surfer's power. Even this was inconclusive since the Surfer had been substantially depowered prior to getting his own comic.
The Sub-Mariner is often forgotten in these calculations but, when in water, he can take on anyone. He shared Tales to Astonish with the Hulk for a few years. The last issue had the two squaring off against each other. The Sub-Mariner actually won although he did it by swimming fast enough to create a vortex around the Hulk. The Sub-Mariner had no trouble overpowering the original X-Men but his fights with Iron Man and the Thing were less definitive.
Under Stan, when unequal heroes fought you usually got a definite winner. This often happened in the weaker hero's comic where just surviving the stronger hero was a feat. I already mentioned the Silver Surfer and Thor and the X-Men meeting the Sub-Mariner. Spider-Man spent his first encounter with the Hulk dodging blows and trying to stay alive. Daredevil threw everything he had at the Sub-Mariner and failed miserably (but the Sub-Mariner departed out of respect for a valiant foe). Giant Man and the Wasp managed to survive the Hulk at the beginning of his run in Tales to Astonish. On the low end, Daredevil was no match for Spider-Man.
A strange thing happened as other writers took over from Stan. When two heroes fought, the outclassed hero usually won, usually by a trick. Both the Invisible Girl and Ghost Rider beat the Hulk by cutting off his oxygen. In an Old Avengers vs the New Avengers annual, Captain America, the Black Panther, and Hawkeye all beat Thor, the Hulk, and Iron Man. At one point in the 1980s, the Thing mutated into a stronger form while the Hulk reverted to a weaker gray version. When they fought, the Hulk won anyway. He outsmarted the Thing!
Marvel seems to have a new editorial policy. Thor is the most powerful. Earlier this year he smashed Iron Man's armor. More recently he fought the Red Hulk. It was clearly established that the Red Hulk is the stronger Hulk (although he overheats if he gets too mad). He also admitted that he was likely to loose a fight with Thor.
This matches an early letter page where someone asked who would win - the Hulk or Thor? Stan(?) answered that he suspected that Thor's hammer gave him the edge.
I'm leaving
the Sentry out of all this. His powers are too poorly defined and I've missed most of his appearances.
Of course, defining the most powerful hero in the Marvel universe is just an intellectual exercise. There are several beings who are vastly more powerful than any hero. These include the personifications of space and time. Once anthropomorphic projections of the universe enter the picture, everyone else is outclassed.
