I used to say Armstrong, I used to say Williams. Difficult choice. Williams' style is just about perfect; though he may have stalled against more technical boxers, he was truly lethal if the opponent came to him. His hooks and uppercuts were accurate, fast and powerful.
At the same time, there's no real style to overcome Armstrong, who I'd say was the most formidable physical force of all time. Williams' method is about closest. Armstrong was probably better (and the best ever) at 126-130lbs, but against lightweights he did almost as well.