At a certain weight? Micheal Spinks is undefeated as a Light Heavy. I don't think Michael Moorer was ever beaten as a Light Heavy, not to mention he knocked everyone he ever fought at that weight out. Has Rob Calloway ever lost as a cruiserweight? I'm only aware of him losing to heavy weights. Bob Foster, maybe?
Roy Jones Jr. was unbeaten at 160, and 168. Marciano never lost at heavyweight. Julio Cesar Chavez never lost at 130, nor 135. Evander Holyfield never lost at cruiser.
Alexis Arguello At 130, Carlos Zarate at 118, Ezzard Charles at 175, Robinson at 147, Morales at 122, Holyfield at 195. Sanchez at 126, Camacho at 130. Mayweather at 135.
Wilfredo Gomez was never beaten at 122 in what must have been close to 35 fights. I am pretty sure that after drawing his first ever fight at 122 every guy thereafter was stopped. Incredible.
as good as duran was he did lose one fight and light weight sven ottke from germany was never beaten jimmy barry an old time fighter ibf world champion terry marsh from europe