I think what defines a fighter as great is how he trains for a great performance and how many times he comes forth to do it again and the quality of his opponent. How good was his opposition? Enough to have many great moments where his performance is recognized as great. After all you are only as good as your last fight until you are not able to do it any more. Sadly enough age, eroding of skills and finally desire cause them and us to remember the good performances. Ali was once called,"A thinking man's fighter." That also made him great.