If there's one thing Scarlet Witch had truly become.
And things only got more complicated along the way. In particular, the addition of Agatha provided a particular point of confusion up until the show's finale, as it seemed that not only had the witch managed to outsmart Wanda, but that in doing so she may have been more powerful than the MCU hero. But these lingering doubts were swiftly put to rest by the conclusion of WandaVision, which shows that Wanda is not only more powerful than Agatha, but also may have finally gained the title of most powerful MCU hero.
This is because the fight between Sorcerer Supreme suggests she is indeed at the least the most powerful magic the MCU has shown thus far - and is likely to only grow stronger given she now has the tools to learn about and hone her abilities. At least, this certainly seems the case, given Wanda is shown in the post-credits having either split into two separate versions of herself, or being able to keep up an illusion of herself while focusing on something else entirely. Either way, it's a clear sign that Wanda's powers have evidently grown - potentially to the point she is able to split her physical form and her astral form altogether.
It's also worth addressing that Wanda's powers have likely been suppressed throughout the franchise thus far by an unlikely source - Wanda herself. WandaVision makes it clear that Wanda has been dealing with a lifetime of barely suppressed trauma, and not being able to be in tune with herself in this manner appears to have been repressing her own abilities. This would explain why her grief-stricken breakdown not only crafts her the life she'd wanted for so long, but also seems to do things that would have seemed previously impossible for the Scarlet Witch. With this emotional suppression finally somewhat alleviated - even if she is likely still clinging on to the somewhat worrying hope of being able to rescue her twins - it seems that Wanda's full potential became more accessible for her to use.
Wanda had long been in the roster of most powerful MCU heroes since her introduction into the franchise, but her abilities had seen little real fanfare, in no small part because it was the belief of many that having an "overpowered" character on the team of heroes would remove the suspense from many of the battles. But with the opposing threats only increasing in scale with every new film - and with it being clear the Scarlet Witch's powers are just as capable of hindering as helping if not used right - the decision to boost her magical prowess significantly through WandaVision is one that feels both well-timed and well thought out.