If the Avengers are Earth's Mightiest Heroes, then Thor and Hercules are Earth's Mightiest Avengers. But which of the two powerhouses is mightier? The two have fought countless over the years and have whole histories of their own, but somewhere in all that lore, there's enough to separate the worthy from the worthiest.
THE MYTHOS OF THOR AND HERCULES
There is a lot of history where both Thor and Hercules are concerned, both in and out of the Marvel universe. Naturally, in the real world, the two beings are figures of Nordic and Greek mythology, and in the context of Marvel 616, they are real figures who inspired those myths. Thor's Marvel history dates back a few years prior to Thor's, and from the outset, he established himself as the mightiest Avenger. He could hurl tanks cross-country and lift skyscrapers, and as time went on he was leveling mountains and breaking worlds apart with the aftershock of his fights.
Not to be outdone, Hercules debuted as the Prince of Power and was sure to start gathering some pretty impressive feats himself. Hercules' most famous feats from mythology exist in his 616 canon, meaning he's held up the sky itself within the reality of Marvel Comics. Even more impressive than that, he once beat the Hulk villain Abomination in a single punch -- and the beast is regarded to be stronger than Hulk himself.
CLASSIC CLASHES
During the Silver Age of comics, Marvel would routinely find fantastical ways to up the ante and make their annual titles bigger events than the heroes had ever faced before. For a comic like Thor's, it could be hard to achieve such heights, so for Journey Into Mystery Annual #1, Marvel fans got their first look at what a Thor vs. Hercules matchup would look like. It proved a classic, but if there's anything Marvel knows how to do it's whetting an appetite. That first fight ended in a draw, with both gods being evenly matched before Zeus intervened.
It was far from the last time Thor's Mjolnir tested the mettle of Herc's adamantine mace. Their next fight in Thor #126 had massive collateral as they hurled semis, dump trucks and even whole buildings at each other, but in the end, it was Herc who came out on top. Thor seemed to get the upper hand in their next bout in Thor #221, but that fight ended without a clear victor like most in the Silver Age.
MODERN MATCHUPS
Although the matchups between the two continued to end pretty conclusively, Thor seemed to start to get the upper hand in the modern-day. Since both characters are heroic and even their arm-wrestling matches have thrown the planet out of orbit it's rare they the two will ever go all out, but as recently as Thor: Blood Oath, the God of Thunder started gaining an edge by using his lightning. Herc thinks it's cheating, but some may just call it a sensible move.
So which Avenger is truly the mightiest? The record would seem to lean towards Hercules, as his decisive win in their second match is one of the only times that outside forces or underhanded methods didn't intervene. Such outside forces would seem to complicate the comparison, but they actually help when considering one iconic storyline. During Marvel's Civil War, Thor's clone Ragnarok made a mockery of his reputation. To defend his old sparring partner's honor, Herc went all out and caved Ragnarok's head in with his own hammer. So, there you have it: when Herc's not holding back, he really is the God of Strength.