Darth Nihilus from Star Wars Knights of the Old Republic 2.
He hungered for the Force for such a degree that he drained entire star systems of it, killing billions of people at once.
I was going to say this.
Don't know if he satisfies the canonical part of the topic title anymore
Didn't Kreia explain that power isn't the right word? Nihilus was a slave to his hunger.
I mean that's true, but he still seems objectively more "powerful" than any other character in the game, and probably any Star Wars lore. You are only able to defeat him through the poorly explained plot armor of your character being a unique "wound" in the Force.
If that's how you want to define it. Nihilius didn't seem to have power over his hunger. Sometimes I wondered if he was possibly a fate of Meetra. ---