While they've fought dozens of times in the comic book Marvel Universe, Thor and the Hulk have fought twice in the Marvel Cinematic Universe. The first time was in The Avengers, during a brief fight fought when the Hulk went wild. The second was in Thor: Ragnorak, where the two fought on Sakaar's battle arena in the Contest of Champions. The second fight in particular ranks among the most awe-inspiring fights in the MCU.
However, that fight deserves a worthy sequel. Both Thor and Hulk have even further levels of power to reach if they are to match their comic counterparts. Thor could become King Thor, bestowed with the full powers of Odin in a prophecy that's already starting to come to pass in comics. Meanwhile, Hulk has the Maestro, an all-powerful version of Hulk from an alternate future where he reigns supreme. If these two supremely powerful characters fought as the MCU embraces the possibilities of a multiverse, it could reshape the MCU as we know it.
Who is Maestro?
Maestro is Hulk from an alternate future, where he reigns supreme. As Peter David and George Perez revealed in Hulk: Future Imperfect, this Hulk is driven to the point of madness after a nuclear disaster. The radiation of his nuclear holocaust multiples the Hulk's strength, grants him the intelligence of Bruce Banner, but also drives him into an unstable mental state. Marvel's main Bruce Banner believes this insanity contributes to his incredible strength, since the Hulk continually grows stronger the angrier he is.
Despite his advanced intelligence, however, Maestro has limits. He cannot invent technology, like Bruce Banner. When Spider-Man 2099 reached Maestro's world, Maestro attempted to force the time-traveling hero to repair his time travel devices, all in order to allow Maestro to conquer other worlds.
Maestro rules his future with an iron fist, but is ultimately defeated when the present-day Hulk travels to his time and sends him back in time to the Hulk's gamma radioactive origins. However, Maestro is still stronger than Hulk, managing to actually pummel him into submission due to superior strategy and power. Few who face Maestro have the power to overwhelm or defeat him, making him one of the deadliest figures in the Marvel multiverse.
Who is King Thor?
In another possible future created by Jason Aaron and Esad Ribic in Thor: God of Thunder #1, Thor sacrifices his eye in order to defeat Malekith the Accursed, and, in the process, gains the power of the All-Father. Odin grants Thor his Odin-Force, now re-named as the Thor-Force. Given this, Thor becomes as mighty as his father -- now ruler of Asgard itself.
King Thor is as powerful as Odin -- and, in many ways, just as cynical and world-weary as well. King Thor has been able to topple entire armies that come against him. However, like Odin, his colossal power comes at a cost, requiring him to enter intense periods of rest in order to recharge.
King Thor is missing an arm and an eye, but has replaced his missing arm with that of the Destroyer. Also, since King Thor is seemingly an older version of Marvel's main Thor, he would cease to be if modern Thor was ever killed, since this will retroactively erase him from existence.
Despite these shortcomings, King Thor remains a supreme force of power. He even managed to fight Old Man Logan empowered by the Phoenix Force down. The close fight ended up going to Old Man Phoenix, but considering that the Phoenix is a literal embodiment of cosmic power, this is just proof that King Thor might just be Marvel's mightiest god.
Could This Happen in the MCU?
MCU Odin is never shown to have any phenomenal powers beyond those of the other Asgardians. While he needs to enter the Odinsleep during Thor, never after that does he demonstrate powers equal to his comic counterpart. When Thor inherited the throne of Asgard, neither he nor Valkyrie ever gained the powers of Odin as a result. They simply take the political position without any question. By that logic, if anyone would be filled with the might of Odin, it would be Valkyrie instead of Thor.
In contrast, the Hulk has been coming closer and closer to reaching his Maestro counterpart. In Avengers: Endgame, Hulk and Banner's personas blend into one perfect version: Professor Hulk. With the multiverse being further explored in Phase 4, it seems incredibly likely that Maestro can descend from a dark future and cause pandemonium to the MCU.
With Thor: Love and Thunder bringing our Thor farther from the Asgardian roots of King Thor, it seems increasingly unlikely we will truly see King Thor in his glory -- beyond, of course, the King Thor we already saw in the MCU: duel-wielding Stormbreaker and Mjolnir while trading blows with Thanos. There's always the possibility that time-travel or more multiversal shenanigans could bring that Thor and the Maestro together.
If They Fought?
If King Thor and the Maestro fought in comics, Thor would win. While Maestro has killed plenty of heroes, King Thor is just on another power level, where the forces of the universe bend to his will.
However, things could play out differently if the MCU equivalents of those characters fought. While King Thor was strong enough to wield both Stormbreaker and Mjolnir, Thor maintains a mostly stable, mighty level. Since a regular Hulk already defeated the God of Thunder in Thor: Ragnorak, Thor, in either of his forms, wouldn't stand a chance against Maeastro, especially considering the Hulk's intelligence and cunning. All of Thor's powers over lightning would only do so much against the Hulk with his regenerative abilities, especially if Hulk figures out a strategy to counter the God's lightning powers.
Maestro would be on par, no doubt, with Thanos. A full squad of powerhouse Avengers would probably have to work together to take out the world-conquering rage monster. While there's no guarantee about a fight like this in the MCU's future, let's hope that it won't be too long before Thor and Hulk's next on-screen brawl.