Marvel mentioned who the strongest Avenger was way before the topic came up as a joke in the MCU.
There is a consistent joke, or rather a playful argument, that is bantered between all the Marvel Cinematic Universe’s Avengers over which of them is the strongest on the team. The banter is mostly started by Thor and perpetuated by Iron Man, but it is funny and has fans wondering who the strongest Avenger would be. A little joke in this regard was added to Thor: Ragnarok.
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Thor vs Hulk

In the film, Hulk and Thor find themselves trapped on a far-off trash planet called Sakaar created by the Grandmaster (an ageless Elder of the universe who is obsessed with making lesser beings fight one another), and they are trapped in an arena where they must fight one another. There they have access to the Quinjet, which is how Hulk arrived on Sakaar in the first place, after the events of Avengers: Age of Ultron.
Thor tries to activate the Quinjet’s controls using the credentials of the ‘Strongest Avenger’ after trying his name several times and is hilariously denied. Bruce Banner logs into the Quinjet simply with the name ‘Banner’ and the computer responds “Welcome, Strongest Avenger” which is made even funnier by the fact that Tony Stark is the one who programmed the credentials into the Quinjet and the one who perpetuated the argument amongst the Avengers.
According to fan opinion, however, the strongest Avenger is the Scarlet Witch, followed by Doctor Strange, then Ant-Man, Captain Marvel, Thor, Shang-Chi and then only Hulk, which means that he is considered not only two ranks below Thor, but that even Thor is not the strongest in fans eyes.
Fan opinions may be skewed by the latest events of Doctor Strange in the Multiverse of Madness, however, Marvel has previously named an Avenger as the strongest, and it isn’t Doctor Strange, the Scarlet Witch, Thor or Hulk.
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Who is the Strongest Avenger?

Looking to the inspiration behind the Marvel Cinematic Universe, in the comic Avengers #36 created by Roy Thomas and Don Heck, Scarlet Witch and Quicksilver have been kidnapped by an evil alien warlord named Ixar — who is seeking to capture all the Avengers to copy their powers and conquer Earth-616 (the universe in which majority of the MCU takes place). Ixar sets up a trap for the Avengers and almost succeeds in capturing them when they stop his spaceship looking for their captured teammates.
Bombarded by an army of androids, it seems all but hopeless for the Avengers until they are single-handedly saved by whom the Marvel comics have named the strongest Avenger: Ant-Man.
Ant-Man, Hank Pym at the time, was working under the alias Goliath at the time because of his impressive ability to grow massively or to shrink until he was almost invisible using Pym Particles. In this issue, Goliath saves the day and is referred to as the strongest Avenger in the comic. This is not a title that was bestowed in Hulk and Thor’s absence, as they were all a part of the Avengers at the same time, which makes the title genuine.
In the comics, Hank Pym claims the title of the strongest Avenger because of his sheer size and strength, attributes that the MCU’s Thor and Hulk can easily trump. In the MCU, Ant-Man isn’t even Hank Pym anymore, as the mantle has been passed down to Scott Lang, who seems to pale in comparison to the massively strong Incredible Hulk, and the God of Thunder, and yet he may still claim his title as the strongest Avenger yet.
Lang is only beginning to learn how to operate within and manipulate the Quantum Realm, an untapped energy source that can even be utilised for time travel. If Lang were to learn to be able to stretch that manipulation to new heights, he could reclaim his title as the strongest Avenger, although Banner might always own the title thanks to the fact that Stark is no longer around to reprogramme their equipment (ouch).
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Pym was always a favorite of mine, a complex
and complicated character. His scientific side
was always fascinating. Hope he gets plenty
of respect in next phase!