The actual strongest Avenger is neither Thor nor the Hulk, but Hercules, and the Prince of Power has proven it by beating them both many times.
The title of strongest Avenger is often contested among powerhouses such as Thor and Hulk, but it actually belongs to someone else: Hercules, the Prince of Power. Even the God of Thunder and the Jade Giant would both have to agree with that.
Hercules is one of the many Marvel heroes based on characters from classic mythology, and after Thor he is the most successful one. He debuted in Journey into Mystery Annual #1 in 1965, where he immediately goes toe to toe with Thor, with the battle ending in a draw (an impressive feat, considering how powerful the son of Odin is). After that, Hercules' path crossed that of the Avengers several times until he was made a full-time member. Herc's love for adventure and his desire to test his strength in battle made him the perfect fit for the Earth's Mightiest Heroes and his tenure with the group lasted for a very long time, until the Onslaught event and the following Heroes Return in the 1990s. Herc is still a trusted ally and friend of the Avengers, but he follows his own path, which led him to associate with Amadeus Cho and, more recently, with the Guardians of the Galaxy.
Hercules and Thor are friends and battle brothers, but they have also clashed many times, as superheroes tend to do, and gods even more. Despite Thor's incredible powers, Hercules has proven to be his match, and their fights usually end in a draw. In Thor #126, the two gods clash and it is Herc who comes out on top (but Thor has his power restrained by Odin). In Thor #221, Thor initially seems to have the upper hand, but that fight ends without a clear victor. However, when Loki is looking for a warrior strong enough to beat his adoptive brother, his first choice is Hercules. More decisively, in Thor #356, Hercules takes over the title for one issue, and tells the story of how he dominated his last fight with the God of Thunder (a tale he later reverses, but only to please a young Thor fan).
Hercules has also had his good share of fights against the Jade Giant. The two actually have a lot in common: both lost their wife and children (Hercules in ancient Greece at the hands of Hera, Hulk on the planet Sakaar), and their powers work in a similar way. Hercules is the god of raw strength and as such his physical strength seems to have no upper limits, as clearly stated by many writers. Hulk's strength, on the other hand, increases with his anger, again with potentially no limit. A brief fight during World War Hulk ends with Hercules letting Hulk hit him without reacting just to convince him he wants to help (and this is the strongest Hulk ever).
A full confrontation happens in the dystopian future setting of Maestro. This alternate version of the Hulk - who maintains Bruce Banner's intellect - awakens in a post-nuclear wasteland and comes to blow with its ruler, Hercules. The fight is totally on Hercules' side, who forces Hulk to beg for mercy. Even if this version of Hulk is not very good at getting angry, it is also true that Hulk's "angry form" is an upgrade of his basic one. If the same principle were to be applied to Hercules, his "upgraded form" would be that of a fully-fledged god like his father Zeus, who completely destroyed Hulk in their only fight (Incredible Hulks #622).
Writers often take artistic licenses when dealing with Hercules' strength, as Stan Lee himself pointed out at one time. However, it is undeniable that Hercules has bested both Thor and Hulk, proving he is indeed the strongest Avenger.
