Based on implied power? The Ancient One. It's implied in Dr Strange that she spent centuries dealing with magical cosmic threats.
Based on available feats? Agatha, unless the Ancient One decides to take it to the Mirror Dimension, in which case Agatha probably gets squashed by a building. People hype Agatha for what she did with Wanda, but she had to use prep that stripped Wanda of her powers and hostages to gain the upper hand.