Well, greatest is a very subjective term. It's hard to measure the competitiveness of eras against each other, and often, the "golden age" fighters had pretty records which were stacked with local bums and then buffed up with some more famous names. Then you have to take into account things like fame, weight, fantasy matchups, skills etc... THat said, there's generally a pretty solidly understood bunch who are pretty clearyl above the rest. They're all pretty comparable and fighter A isn't really clearly better than fighter B in this elite upper tier.
It's generally acknowledged that this elite list is composed of: Orlin Norris, Wiliam Joppy, Tony Tucker, Howard Eastman, Fres Oquendo, Gerrie Coetzee, Joe Hipp, Robbie Sims, Bobby Czyz, Carl "the cat" Thompson, Herbie Hide, Axel Schultz, Francesco Damiani, O'Neil Bell, Antonio Tarver, Robin Reid, Sharmba Mitchell, and Clinton Woods. Anybody else is an imposter to the throne.