Scarlet Witch is the most powerful Avenger because she possessed extremely strong powers which were only amplified by her exposure to the Mind Stone. Wanda possesses the powers telekinesis, energy manipulation, and neuroelectric interfacing, allowing her to read minds and transport people into their nightmares.  

In the new show Wanda Vision, she is able to go create her own dreamland. After being overcome with grief after the loss of her husband, Vision, power bursts out of her. She creates the hex and everyone within Westview becomes a part of her dreamland. Within the hex everyone becomes a part of Wanda’s sitcom. After Wanda discovers she can create matter out of nothing she creates her two children and Vision by using these powers. 

In the climax in Avengers Endgame Wanda is able to face off with Thanos, who had all the infinity stones. Wanda is able to crush Thanos with his Armor, the only reason for her losing is her anger. Endgame also showed the Scarlet Witch in the infancy of her powers, she had not yet reached her full potential. Marvel’s Chief Creative Officer, Kevin Feige confirmed that he believed Wanda Maximoff was the strongest Avenger after she almost killed Thanos in Endgame. In the sequel to Doctor Strange Wanda has harnessed her powers and is able to bend reality and kill another multiverse Avengers team. Before this sequel she didn’t know how to handle her rage and the Darkhold, an ancient book of sins, helps her to use that rage to become more powerful. 

While the Scarlet Witch contains some of the strongest powers created in the MCU series and is strongest magically, Thor is  strongest physically. Thor is a God and believed by many to be the most powerful Avenger. He was born on Asgard, home to the Gods, and is the son of Odin. Thanks to Mjolnir, Thor’s hammer, he was blessed with powers over elements such as lightning and thunder. Along with these powers he has superhuman strength, speed, agility, and durability. 

Scarlet Witch continuously becomes more and more powerful throughout the series as she learns to control her powers. Once gaining full control and reaching her full potential she becomes the most powerful Avenger. Although Thor is a strong rival for the most powerful Avenger, Scarlet Witch’s powers outnumber his strength.