My 5) Bob Fitzsimmons (crazy, inhuman power, fighting heart) Sam Langford (almost as wicked power as Fitzsimmons, a natural pursuer and finisher) Harry Greb (you have this almost incomparable resume, especially with somewhat limited training, without having some of the greatest natural talent ever) Sugar Ray Robinson (speed, power, vision, boxing IQ, heart) Ali (ridiculous speed/size ratio, chin, heart) close call on Duran (power, speed and instincts)
Decent list. Much depends on what exactly the threadstarter had in mind. As a general rule, I would probably never include Langford, great as he may have been on account of the lack of ability to watch a decent portion of his body of work.