What boxer has the best win above their natural weight class?

Duran moving up two weight classes and going toe to toe with one of the most dangerous fighters of that era, awesome display in Montreal.

 

Duran actually did it twice.
In Montreal against the 2nd best welterweight in history in Ray Leonard. WHEN BOTH FIGHTERS WERE PRIME. ( Unlike the smoke and mirrors of Mayweather Jr. And his "wins")
Duran pulled a double when he somehow pulled off the upset of Iran Barkley, ironically right after he beat Thomas Hearns.
A legendary feat in itself when looking at all of Barkley's advantages. Including size, power, strength, age, and coming off the biggest win of his career ko'ing Hearns.
Hearns wins over Virgil Hill was very impressive.
But Hearns was a Lt.heavy that could disguise himself as a welterweight/ Jr. Middleweight when looking at his freakish physicality.
Jones Jr beating John Ruiz was impressive. Though Ruiz was tailor made for Jones, and fought the way Jones needed him to fight for him to win.
Emile Griffith victory over Dick Tiger was also impressive.
But no victory I can think of fits the criteria of this subject better than "The Brawl in Montreal ". Especially when considering both fighters were close to or were prime.

 

Harada beating Jofre wasn't to bad
Fitz beating Corbett was ok
Armstrong - Ross
Armstrong - Ambers
Greb had oodles

Most from the distant past are invariably forgotten as the 70's forward era dominates. There's so many pre 70's one can't count them.

 

Duran is the first to come to mind, as it’s really hard to get better than beating a 80’ SRL.

Ezzard has a bunch of quality wins above his natural division.

 

A few worth mentioning:

Thomas Hearns over Virgil Hill: Especially notable if we’re going to factor in how far above natural weight class — welter vs. light heavy.

Ray Leonard over Marvin Hagler: Many argue Marvin as one of the top two or three ever at middle.

Michael Spinks over Larry Holmes: Nuff said.

Floyd Mayweather over Canelo Alvarez: Practically a shutout over the next big thing who was already in his early prime.

 

So many but too me, going from 160 to 175 or 175 to Heavyweight are particularly impressive because of the gap in weight. So many good ones, below are just a few.

Tiger vs Torres-- Tiger didn't even weigh in the 170's when he won the light-heavyweight title.
Hearns vs Hill-- Hill was a legit champ at 175 and tough matchup. One of Hearns greatest wins.
Ezzard Charles was a natural middleweight that became deadly at 175 and Heavyweight.
Spinks and Moorer moved to Heavyweight and not only won lineal titles against great fighters but also had the power to hurt some very big Heavyweights.
Bhop going from a dominant reign at 160 and shutting out Antonio Taver was impressive as hell.

 

Most here have mentioned great examples.

I will also add:
Santos Laciar over Gilberto Roman
Julio Cesar Chavez over Mayweather part 2 and Taylor part 1
Henry Armstrong over Barney Ross on paper at least, I know Ross was pretty past it by then.
Chapo over Loreto Garza
Holyfield over Douglas, over Foreman, Bowe , Tyson and Moorer
Chacon over Limon 4 and over Boza Edwards 2
Wasnt Jimmy McLarnin a flyweight once? Cause he beat Pancho Villa but Im not sure but if he was then his win n fight two versus Barney Ross

 

Just adding a few more

Ruben Olivares (the once great bantam and feather) knocking out the rock-jawed and future lightweight champ Jose Luis Ramirez in 2 rounds.

Flash Elorde (Jr. lightweight champ) outclassing and dropping (twice) the former and future lightweight champ Ismael Laguna.

Ike Williams (lightweight champ) had two great wins at welter. Beating top contender and future welterweight champ Kid Gavilan on a close decision while dropping him for one of only two knockdowns Gav suffered in his career. Also, Ike knocked out future welterweight champ Johnny Bratton in 8 rounds, breaking his jaw in the process.

 

Duran beating Leonard and Barkley is larger than life. It shouldn't have happened. Leonard was a pretty-faced killer. Ali-like grit shown against Hearns, and Duran tempered those fangs like some kind of chess-playing barbarian king—inside and outside. Barkley size and age difference was bizarre as well.

Mickey Walker and Barbados Joe Walcott had battered or beaten men they should not have interest being in the ring with.

Armstrong really did not care about bringing the proper weight to the higher divisions he set to conquer.

 

The hardest wins are those that are achieved by beating guys who are naturally much heavier.

Micheal Moorer defeating Evander Holyfield.
LHW -> HW

Sam Langford beating Joe Jeannette, Philadelphia Jack O'brien, Sam McVea, Kid Norfolk and Harry Wills.
MW -> LHW, HW

Thomas Hearns beating Virgil Hill.
JMW -> LHW

Roberto Duran defeating Iran Barkley.
LW -> MW.

 

Joe Walcott's win over Joe Choynski, when outweighed by 16 pound (147 vs 163).
Not a win, but Charlie Mitchell's draw with John L Sullivan, when outweighed by 34 pounds (166 vs 200).

 

Camacho had some great successes all the way up throughout his career.

 

Grebs win over Tunney ranks up there as well.

 

Moorer's not smaller than Holyfield. He moved straight up from LHW to heavy at 22, Holyfield went from LHW to 190lbs cruiser at the same age.

I'd also argue Hill wasn't naturally a much bigger man than Hearns, maybe 7 or 8lbs but not much more. Tommy was just more willing to shed more weight to get in with the big bucks.