Thor's Hulk Form Finally Made Him the Strongest Avenger
While Thor has long been irritated by the fact that he's technically not the strongest Avenger, he at least got to claim that title for a little while when he combined his power with the Hulk's, assuming a staggering new form.
The transformation took place as part of the 'World War Hulks' story, in which the Red Hulk's storehouse of stolen power was distributed among Marvel's heroes, causing them to Hulk-out and assume gigantic new forms that unlocked the full extent of their powers. But while heroes like Ms. Marvel, Cyclops, and the Human Torch all got god-tier upgrades, the strongest among them was Thor Odinson, whose control of his diving lightning only increased along with his muscles.
In Jeph Loeb and Ed McGuinness' Hulk #21, Red Hulk (aka General 'Thunderbolt' Ross) falls afoul of his former allies in the Intelligencia. This group of Marvel's smartest villains - including MODOK, the Leader, and the Mad Thinker - intend to take over the US, siphoning off Red Hulk's collected energy and using it to empower a new army of Hulks. Thankfully, several Marvel heroes are also affected, inheriting the traditional lack of emotional regulation that characterizes Hulks, but capable of pushing through the fog of rage to hold back the Intelligencia's army. Each hero assumes a towering form, but with the drawback that the incredible power will burn out their bodies in twenty-four hours.
After Thor beats down the Red Hulk, he teams up with Spider-Man to battle the Intelligencia's army, but gets distracted trying to control the Hulked-out Peter Parker. In World War Hulks: Spider-Man vs Thor's main storyline (from Kieron Gillen, Jorge Molina, Paul Pelletier, and Ronan Cliquet), Thor attacks Spider-Man, trying to persuade the distracted Peter Parker to join the battle, only for Mjolnir to suddenly grow heavier, implying Thor is no longer worthy. Being a Hulk brings out the worst in Thor, returning his famed arrogance and bloodlust, but he eventually remembers a lesson from his childhood, when Odin tried to persuade him to treat the bookish and unadventurous Loki as a brother. Thor's hard-won nobility is able to give him control over his Hulk form, and he lifts Mjolnir and takes a pivotal role in suppressing the Intelligencia's Hulk army so the genius villains can be defeated. But while Thor's better qualities win out, he is sure to note to Spider-Man that while the Hulk may have been able to beat him in the past, this upgrade officially makes him "the strongest one there is."
Rage has long been the thing that makes Hulk so strong, and Thor has more than enough to spare in his ultra-powerful gamma form. However, what 'World War Hulks' did was reveal that while many fans think of Thor as a blunt instrument, his nobility and humility took multiple human lifetimes to achieve, and they're essential to his role as a true hero rather than an arrogant bully. Thankfully, Bruce Banner is ultimately able to absorb the radiation from the assembled heroes, preventing their new powers from destroying them.
Thor has gone through many changes as a Marvel god, from becoming King of Asgard to slaying Galactus single-handed. And yet in terms of pure physical strength, 'World War Hulks' finally gave him a form that could out-muscle the Jade Giant. Thankfully for Thor's ego, he got to spend at least one adventure as the true strongest Avenger, but he also showed that he possessed the psychological fortitude and personal nobility to take the Hulk's rage and channel it towards good - a feat many other heroes couldn't manage.