Greatest of all time is hard to state because there have been a couple of legends who can wear that hat... but each for different reasons.
1. Holyfield - Why? Because his fights were among the most exciting fights in boxing history. Damn near every one of them was a classic. Secondly, he was never a true heavyweight. He was a cruiser who buffed up to fight with the big boys. That is an incredible accomplishment. Holyfield heart and soul is what great boxing is about.
2. Ali - definitely ready to wear the crown as the all time best boxer; unfortunately, most of his fights were dull as hell. Rope-a-dope ruined boxing for years. Yes it was a winning strategy, but it abandon the concepts of "courage" and "champion" and turned boxing into something infinitely less noble.
3. Lewis - great fights every one. He defined "champion." He was one of the hardest hitters boxing has ever seen, a scientist in the ring, and fast fast fast. He met every opponent with everything he had.