Back in 2017, Thor: Ragnarok featured a joke in the form of a security question on the Avengers Quinjet. It simply asked who the strongest Avenger was, and Thor arrogantly assumed it to be himself before he was quickly shot down. It was only later, when Hulk transformed back into Bruce, that the answer to the question was revealed to be Bruce Banner. Thor attempted to brush it off as a joke on Stark's part, but that joke has spiraled into years of fan theorizing and discussion over who truly is the strongest Avenger in the Marvel Cinematic Universe.
The real question, however, is if such a title even matters. Being the "strongest Avenger" itself is such an arbitrary and vague statement and has only ever been made as a joke in the context of the MCU films, even as recently as the Thor: Love and Thunder trailer, where the Thunder God is wearing a cap proclaiming himself the strongest Avenger. Not only does the question of the strongest Avenger not really matter, but the filmmakers know this. Most of the time, they decide to be quick and loose with the idea and simultaneously have a bit of fun with it.
The truth is that any Avenger could be argued as the strongest in the MCU. If one is asking about physical strength, then the answer is probably the Hulk or Captain Marvel. Strength of their powers? Scarlet Witch or Doctor Strange. Strength of will? Well, the argument could be made for Captain America, Iron Man, Spider-Man, and countless others. The joy is that any character could be argued as being the strongest, which means that any fan can argue that their favorite character is the strongest Avenger. By not letting there be a clearly defined answer, fans are free to stick by their favorites. This is even lovingly riffed on during an episode of Wandavision, where Darcy is arguing with Jimmy Woo over which Avenger was strongest during the Battle of Earth.
And even if a fan wants to find out who is definitively the strongest Avenger in a specific category, the MCU is deliberately inconsistent on this front because the focus is always on telling an entertaining and engaging story, not concrete power levels. None of it follows a consistent iron-clad logic because that would be dull, boring, and predictable for the audience.
Just to give a few examples, Iron Man needs the Hulkbuster armor to defeat the Hulk in Age of Ultron and loses in hand-to-hand combat with Captain America in Civil War. Yet in Infinity War, he can hold his own against Thanos for a decent amount of time in a standard Iron Man armor while both Cap and Hulk are easily dispatched by the Mad Titan. Similarly, in that same film, Doctor Strange puts up a strong fight against Thanos while it's clear that, in the same fight, if Thanos got the opportunity, he would have squashed Spider-Man like a bug. Yet in No Way Home, Spider-Man can defeat Doctor Strange, in the Mirror Dimension no less.
The question of the strongest Avenger was brought in as a simple joke in a funny film. It was never meant to spark years and years of theories and discussion. But fans are fans, and fans care -- it's what makes them so great. The answer to who is the strongest Avenger is simple. And it's not up to the movies and TV shows to decide. It is whoever fans want it to be, and no one can take that away from them.