'Sheer power", assuming all tennis balls basically mass the same, comes down to who can make the ball travel at the highest speed. Venus still holds the record for serve speed. And open stance shots let you get the rotation of your legs and hips ahead of the raket, so your whole body is pulling the racket head toward the ball.
So I'd say Venus is still the player capable of hitting the hardest shots. However, that isn't how she plays anymore. Even Davenport hits most shots at less than her full power. The player who's groundstrokes probably average the highest top speed is Sharapova. Of course, I haven't seen much of Mirza.