Holyfield could drop anyone because of his timing. He's a great counter-puncher. I'd give his power a 6 or so.
He had speed, leverage, great timing, and commitment behind that power. It's hard to separate one factor out in the equation. All told, I'd give him an 7. At least.
He had good pop. Not a one punch puncher though. And not up there with the real hard punchers. A six.