While many fans may measure an Avenger's strength by how powerful they prove themselves to be in any given situation, one could also argue that true power is not needing to lift a finger to be victorious against even the strongest villain-which is something that was exhibited by only one MCU Avenger, proving them to be the all-time strongest.
Though it's true that the debate over which Avenger is the strongest has raged throughout the Marvel fandom long before the MCU ever even came to be, the conversation didn't really enter the Marvel Cinematic Universe until the film Thor: Ragnarok. In the movie, Thor feels as though he is the strongest Avenger even over Hulk, though Hulk believes that title belongs to him. Thor then faces even more indignity when the Avengers' quinjet addressed Hulk as the 'Strongest Avenger' and addressed Thor as 'Point Break.' Since the conversation about which Avenger is really the strongest officially entered the MCU in this movie, the debate became even more prevalent within the fandom as some believe Thor did deserve that title with others maintaining that Hulk truly is the strongest-and even some throwing Captain Marvel and Scarlet Witch's name in the ring as well. However, in one of the Avengers' earliest adventures, only one MCU Avenger proves to be the strongest of them all: Black Widow.
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In Avengers #37 by Roy Thomas and Don Heck, the Avengers have been abducted by an alien conqueror named Ixar who is hellbent on stealing the Avengers' powers and using them to take over the Earth. After battling a number of Ixar's android minions, the Avengers finally confront the villainous leader himself and his physical appearance is less than intimidating. While Ixar pretended to be a highly-advanced computerized system, he was really just an old man from another planet nearly identical to Earth who sought to conquer far-off worlds with his heightened technology. When the Avengers tell him to bring them back to Earth, Ixar just laughs in their faces. Since the Avengers made an oath to never kill anyone, and Ixar knows that, the villain decides he doesn't need to listen since there will be no consequence for him either way. However, Ixar wasn't aware of how incredibly hardcore one of the Avengers truly was.
When the other Avengers admit they can't kill Ixar, Black Widow steps up, points a loaded gun to his head, and tells him that she won't hesitate to pull the trigger. When Ixar states that she's bluffing, Black Widow responds by telling him to look deep into her eyes and know that she will kill him if he doesn't do what she says. The very next panel, the Avengers are back on Earth and Ixar is captured, proving that Black Widow was victorious which officially elevated her intimidation level to cosmic proportions. Ixar was from a far-off world that was torn by war, he developed highly-advanced technology way beyond what any scientist or inventor was doing on Earth (including Tony Stark), and he was nearly successful in defeating the Avengers themselves-and Black Widow took him down with a single look.
While it is no secret that Black Widow isn't as mighty as Thor or incredibly strong as Hulk, she did manage to defeat a cosmic-level villain with a mere glance, one who brought the entire Avengers team to their knees which included the likes of Captain America, Ant-Man (Hank Pym), and Scarlet Witch. This is more than any other Avenger can say. Since Black Widow took care of an Avengers-level threat without having to throw a single punch or shoot a single bullet, she reveals what true power actually looks like and in doing so takes the top spot as the strongest Avenger, MCU or otherwise.