certainly not graf. her FH was hard but her BH was just a slice. to be considered you need power off both wings.
seles on the other hand did have power of both wings and would hit the ball hard all day (consistently much harder than graf) but she did lack that snap on the strokes.
when she played graf she often bossed her around with her power ground game but usually needed around 5 well placed bombs to both sides to hit a winner. graf on the other hand would hit winners from nowhere needing only one FH to do the job.
so I would not consider either.
My top picks:
-henin (she was short but very strong)
from the modern girls sharapova and kvitova are also very hard hitting consistently. lisicki can hit it too.
I'm not sure about davenport. she was a very clean flat ballstriker and placed it well. I'm not sure if I would rank her as high as the others although she had deceptive power. because of her long levers and clean, flat striking the balls were a lot harder then they looked.
she of course used much less effort to hit the ball hard then say henin because she had very long arms.