Muhammad Ali was just as exciting out of the ring as he was in it.
At one point he had the most recognized face in the world. He was bigger than the sport. I can't even tell you who the heavy weight champion is today. What also makes a boxer great is having well known competitors. Ali had Joe Frasier, Ken Norton, Leon Spinks, and George Forman. His fights were events! He was not only a boxer but also the best promoter in the game.
After the Muhammad Ali era I'd have to say the Sugar Ray Leonard era was the next most exiting with Tommy Hearns, Marvin Hagler, and Roberto Duran. The eras of Larry Holmes and Mike Tyson simply can't compare. There were no marquee name fights.
By the time Tyson fought Holyfield they were past their prime.
Muhammad Ali's life has mythical elements to it that no other boxer has. His returning to his hometown Louisville after winning the gold medal at the Olympics only to be refused service in a coffee shop, tossing the medal into the river, having his boxing license taken away for 3 years for not going to Vietnam, predicting rounds he'd knock an opponent out, writing poems to promote fights, winning the heavy weight title 3 times, having feisty arguments with sports caster Howard Cosell, and daring to yell he was "the greatest of all time" to anyone who'd listen. The "Ali shuffle" and fighting with his hands down, and the "Rope-a-dope" were copied by many boxers.
No boxer or athlete period will ever capture the imagination of the world the way he did. Love him or hate him everyone heard of him.