Boxing is perhaps one of the hardest sports to measure greatness as its dynamics change from era to era.

Not only that, but how do you compare a heavyweight to a lightweight? It's pretty much impossible.

However, there is the pound-for-pound rankings that could help in this instance, with a lot of people hailing Saul 'Canelo' Alvarez as the current P4P king.

But who is the P4P king in the entire history of boxing?

Yard Barker did their best to have a look at over the 100-plus year history of boxing and rank the best pound-for-pound fighters to ever grace the ring... and here's what they came up with!

25. Sandy Saddler

Saddler is best known for his series of fights with Willie Pep, which lasted four bouts. Saddler finished with a 144-16-2 record and was the only fighter in history to beat Willie Pep when he was at his best.

24. Rocky Marciano

One of the most well-known boxers on the list. “The Brockton Blockbuster” finished his entire career unbeaten and defeated legends Ezzard Charles, Jersey Joe Walcott and Archie Moore during his reign at the top.

23. George Foreman

Became a champion just four years into his career. Was unbeatable until he faced Muhammad Ali, when he was knocked out. That night ruined Forman’s prime and he was never the same fighter after.

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22. Julio Cesar Chavez

Chavez finished with a 108-6-2 record and at one stage was 88-0 and ruled the lightweight division for years.

21. Jack Dempsey

Rose to superstardom with the knockout of Jess Willard, who was at the time a 240-pound monster. Finished with a 55-6-8 record and is an all-time great.

20. Gene Tunney

Tunney finished his boxing career with a 61-1-1 record. He was the one who beat Dempsey to claim his title via unanimous decision and retired after the very controversial rematch.

19. Barney Ross

Ross won three titles in three different weight classes and finished his stellar career with a 72-4-3 record. He was the welterweight champ for years before losing to Henry Armstrong.

18. Joe Gans

A true trailblazer, Gans was the first black American to win a world title. Finished with a 120-8-9 record. Best known for his 42-round slugfest with Battling Nelson for the 135-pound strap.

17. Floyd Mayweather Jr

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Undefeated at 50-0 and is one of the most skilled boxers ever to step into the ring. The only knock on 'Money' is that he refused to take on champs in their prime and opted for easy fights in his late 20s.

16. Pernell Whitaker

“Sweet Pea” is the best defensive boxer to ever live and was untouchable at his best. Held a 40-4-1 record and those draws and one loss are extremely debatable. He was boxing’s best for several years.

15. Archie Moore

“The Old Mongoose” had one of the most unique careers ever. Held a 184-24-10 record with 130 knockouts. Moore did not get a title shot until he was 39 and held the 175-pound class for nine years.

14. Ezzard Charles

Charles beat four 175-pound champions, but never fought for the belt. Arguably the greatest lightweight ever, he finished with an 89-25-1 record.

13. Manny Pacquiao

Pacquiao is an eight-division champ and holds a 61-7-2 record. Fought Mayweather, although Floyd waited until he was well past his prime as he beat him. A historic boxer who deserves all the praise.

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12. Sam Langford

A 167-38-37 career record, but Langford is the lists only non-champion as he never got the chance to fight for a belt. The ultimate what-if athlete.

11. Jack Johnson

The first black man to win the heavyweight belt, finishing with a 77-13-14 record and is one of boxing's most important athletes.

10. Marvelous Marvin Hagler

Hagler was avoided for years and finished with a 62-3-2 record and went on a seven-year title reign. A true working-class boxer whose numbers stack up well in history.

9. Sugar Ray Leonard

An Olympic gold medalist and was the face of boxing after Muhammad Ali. An eye injury robbed him of a top-five ranking all time, still, Leonard’s record finished 36-3-1.

8. Benny Leonard

Was the reigning lightweight champion from 1917 to 1925 and ended his career with 69 knockouts and an 85-5-1 record.

7. Joe Louis

Louis was the longest-serving heavyweight champion. Won the belt in 1937 and defended it a record 25 times. The “Brown Bomber” is truly an all-time great.

6. Roberto Duran

Had a 33-year career in boxing and ended with a 103-16 record. In the 1970s, he went 54-1. He was the man who took Sugar Ray Leonard’s belt.

5. Willie Pep

The only rival to Pernell Whitaker’s GOAT of defense in boxing. Won his first 63 fights and ended with a 230-11-1 record to be one of the best to ever do it.

4. Harry Greb

Perhaps the best middleweight ever. A 160-pound champ with a rugged and dirty style of boxing. Cleaned out multiple divisions in the 1910s and 20s. A 105-8-3 is one of the all-time great records.

3. Henry Armstrong

Had one of the greatest sporting years ever between November 1937 to August 1938. Going 27-0 with 26 knockouts. A 151-21-9 record which includes holding three titles all at once.

2. Muhammad Ali

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A three-time champ, his “Thriller in Manilla” with Joe Frazier remains one of the all-time great fights. A 56-5 record for one of the very best to ever do it.

1. Sugar Ray Robinson

Won the 160-pound title five times and only lost once in 123 fights which is incredible. A 175-19-6 record puts him the Associated Press' fighter of the 20th century.

So there's the top 25 greatest pound-for-pound fighters in boxing history, according to Yard Barker.

It makes you wonder, where will boxers like 'Canelo' feature on this list once they've retired? Would they make the top 25?