First things first, he has 5 losses. A true great boxer will have no losses or maybe 1 or 2, not 5. 5 losses shows lack of discipline and not studying your opponents carefully.
Now lets take a look at his resume
George Foreman - just a big man with big punches but was slow as a turtle. I could probably beat him by jabbing and slipping his punches
Joe Frazier - only been in 37 fights, inexperienced boxer, still an ok fighter
Earnie Shavers - has 14 losses
Ken Norton - 7 losses, and he beat Ali (possibly twice)
I dont see how his resume makes him the Greatest. Maybe Muhammad Ali - the Averagest or The most Average would be a better nickname