White hot Pierce is probably the hardest hitter ever, and maybe one of the best ever period. The level of play and striking she displayed at the 1994 French, and at other places through her career, was just plain scary. That 94 SF with Graf was just amazing and terrifying at the same time...she was phenomenal. Problem is she was never consistant.
If I wanted to pick a consistent hard hitter I would have to go with Davenports ball striking (fitness issues not factoring in). She was consistent and could knick lines with ease. Serena and Venus hit hard but are more error prone. Graf and Seles..both hit hard..but they don't come as high in comparison.