Fans will always debate who is the strongest Avenger and this will always continue. If we consider MCU, Tony Stark is the biggest hero because he ended the big war! But he certainly wasn’t the strongest Avenger. Avengers Endgame clearly set it up between Scarlet Witch & Captain Marvel.
Thor: Ragnarok & Infinity War hinted at Thor’s strength is the most. We saw his strength grow in Ragnarok when he fought Hela and her army. In Infinity War, when he took the whole impact of the star, still he survived! With the Stormbreaker, he almost defeated Thanos. But Thanos proved to be too strong for him in Endgame.
Some might argue that Doctor Strange is the strongest, as he is after all the Master of Mystic Arts and has a lot of powers at his disposal. But, his powers too have a limitation.
The same goes for Captain America. He has come from strength to strength in all the movies. Yes he was worthy & might have defeated Thanos alone. He got the Hammer and struck Thanos with lightning too. But, Thanos got the better of him as well.
Just how strong is she? What’s crazy is that her powers haven’t been fully explored yet. They will develop soon enough as we’ve got WandaVision coming our way. The Disney+ series may actually introduce us with her reality-altering powers. In the comics, she creates an alternate reality called the House of M. She did that because Vision and her kids died at her hands. We can expect her to do the same in WandaVision.
Captain Marvel is strong, but she cannot alter realities. So, Scarlet Witch is crazy strong. The only weakness she has is durability. Rhodey easily distracted her in the Civil War. But, that could be prevented if she constructs a Force-field around her. How crazy is it that Tony thought Hulk to be the strongest Avenger! Of course, with new heroes coming soon, these dynamics might also change.