Agreed closely followed by Pele IMO though this thread has become somewhat vague in terms of relating to the actual title. That said Ali of course won the Heavyweight title a record 3 times and despite his 5 losses in 61 fights was for me certainly "The Greatest" and I also love so many of his quotes
"My toughest fight was with my first wife."
"Frazier is so ugly that he should donate his face to the US Bureau of Wild Life. !"
"I am the astronaut of boxing. Joe Louis and Dempsey were just jet pilots. I'm in a world of my own. "
"I am the greatest, I said that even before I knew I was! "
And my favourite:
"I know where I'm going and I know the truth, and I don't have to be what you want me to be. I'm free to be what I want. "