The Casey Bat from EarthBound had the maximum possible attack power, but also the lowest hit rate, actually giving it lower average damage than other bats available at the same stage of the game.
But what makes her this is her potential; she has some, if not the best, mix-ups and mind-games out of anyone, and her ultra's and supers are the highest damaging in the game, with a maximum potential of near twice the lowest damage and a good 30% higher than the closest competitor.