It depends on what you think is important
Longevity, power, skill how mant good opponents, etc.

For length of career,
1. Joe Louis almost 12 years
2. John L. Sullivan 10 years with bare fists.
2. Larry Holmes 7 years
3 Archie Moore more knockouts than any boxer in history.

Skill:
1.M. Ali
2.Floyd Patterson
3Larry Holmes. best jab ever
4.Jimmy Young master boxer, taught Ali a thing or two.
5. Ezzard Charles
6. Lennox Lewis
7. Evander Holyfield I don't know where to put Jack Johnson, but he surely does belong on this list.