Floyd Mayweather Jr has insisted he won't box again after matching Rocky Marciano's 49-0 record. So which current world champions are in with a shout of building a better legacy than that?
Some big names make the top 10 but there's also some whose level of opposition hasn't afforded them much exposure as yet - starting with the man leading the chase.
1. Cesar Cuenca - 48-0-KO2
IBF light-welterweight champion
With a knockout percentage of just 4%, Cesar Cuenca hasn't been able to win many headlines. The Argentine southpaw only became a world champion in July when he outpointed Ik Yang in China and is currently preparing for his first defence against Eduard Troyanovsky in Kazan in November. Victory for the 34-year-old would see him draw level with Mayweather Jr and Marciano.
2. Roman Gonzalez - 43-0-KO37
WBC flyweight champion
At the age of just 28, Nicaragua's Gonzalez seems poised to breeze past the record. His last defence was some of his most emphatic work yet - he blasted the experienced Edgar Sosa aside inside two rounds. Next up for Gonzales is Brian Viloria on October 17.
3. Wanheng Menayothin - 38-0-KO13
WBC strawweight champion
At boxing's minimum weight, Menayothin is one of the leading stars and at the age of 29, also looks set to challenge for a place in boxing history. Menayothin has never fought outside his native Thailand and makes the third defence of his belt in late November when he faces South Korea's Young Gil Bae in Bangkok.
4. Deontay Wilder - 35-0-KO34
WBC heavyweight champion
America's leading heavyweight seems on a collision course with rising British star Anthony Joshua but, for now, Wilder is miles ahead on paper. The 29-year-old from Alabama has made two defences since dethroning Bermane Stiverne in January and is now set to take on his mandatory challenger Alexander Povetkin.
5. Kell Brook - 35-0-KO24
IBF welterweight champion
Brook chased a bout with Mayweather earlier in the year but in the end had to settle for a defence against Diego Chaves. The Sheffield star won his title by edging Shawn Porter in California in August 2014 and has since defended against Jo Jo Dan and Frankie Gavin.
6. Carlos Cuadras 33-0-1-KO26
WBC super-flyweight champion
With 26 knockouts, Cuadras can clearly punch. One of the many fighting heroes of Mexico, Cuadras made his American debut in November last year when he stopped Marvin Marbait in the sixth in Washington DC. He was last in action in August in his homeland - halting Dixon Flores in five.
7. Gennady Golovkin 33-0-KO30
WBA (Super) middleweight champion
Some say tGennady Golovkin has inherited Mayweather Jr's crown as the best pound-for-pound boxer in the world. Others say that he's in for a shock when he takes on IBF king David Lemieux on October 17. Either way, the 33-year-old Kazakh is an unbeaten fighter with a phenomenal knockout record and is one of the sport's big names.
8. Leo Santa Cruz - 31-0-1-KO17
WBA (Super) featherweight champion
Arguably Mexico's leading boxer at the moment, Santa Cruz is now a three-weight world champion. His biggest showdown yet came in August when he took on his compatriot Abner Mares in California and emerged with a majority decision victory in only his second fight as a featherweight.
9. Scott Quigg - 31-0-2-KO23
WBA super-bantamweight champion
The fighting pride of Bury, Lancashire is still tangled up in a lengthy stand-off with IBF champion Carl Frampton over a domestic showdown. That shouldn't detract from the fact that the 26-year-old is in the form of his life at the moment, though - he blasted aside former champion Kiko Martinez in the second round in Manchester in July.
10. Sergey Kovalev - 28-0-1-KO25
WBA (Super), IBF and WBO light-heavyweight champion
The Russian is one of boxing's more exciting fighters to watch and he's failed to knock out only four of his professional opponents. He may be heading for a super-fight against the unbeaten Andre Ward next year and at the age of 32, still has many more big nights left in him.