Pretty Boy and Money are both top nicknames and he damn well lives up to the second nickname. I'd still take Triple-G GGG though. It sounds like a type of explosive or something and he has steel rods in both his arms. I like Keith "One Time" Thurman too, but I'm a fanboi.
"The Machine" by Matthsse, thats obviously the best one out there. Anything that calls itself THE MACHINE is automatically to be feared and gives a since of invincibility. Just look at how Germany's soccer team did with that nickname! Or hell even unofficial it was Drago's nickname in Rocky!!! Personally my favorite would be "The WarMachine". p.s. Bradley's "The Desert Storm" also has a very nice catch to it that only works for him.
Pretty Boy Floyd is one of the best nicknames in the history of the sport Great significance historically (A gangster/bank robber and somebody who escaped prison, badassery), great significance in the fact that he never gets hit, cleanest face. Rivals Iron/Homicide Hank easily. He should have kept it. http://en.wikipedia.org/wiki/Pretty_Boy_Floyd