If you had to name one attribute or skill of these fighters what would it be?

Duran: ferociuousess (see first SRL fight)
SRR: speed
SRL: intelligence
Hagler: Work Ethic
Joe Louis: Straight Right Hand
Jack Johnson: Boldness
Marciano: Guts
Ali: psychological ownership of his opponents
FMJ: Defense
Pac: handspeed

 

Most of mine are intangibles but I think that's usually what separates the very good and the truly great.

Duran: one of the all-time great ring generals
SRR: his ability to knock an opponent out with either hand from seemingly any position
SRL: finding a way to win
Hagler: still fighting like a challenger after becoming a champion
Joe Louis: punching power
Jack Johnson: defense
Marciano: resiliency
Ali: the king of psychological warfare
FMJ: picking the right time to fight his most dangerous opponents
Pac: foot speed

 

Sugar Ray Robinson - Perfect sweet science, had everything
Roberto Duran - wild animal with great technique and defence
Mayweather - Smart and timing
Rocky - A rock with 2 rocks

 

Duran - Skilful intensity
SRR - Basically all strengths and no weaknesses - completeness
SRL - Versatility
Hagler - Dedication
Joe Louis - Offensive ability
Jack Johnson - Ahead of his timeness
Marciano - Single mindedness - completely fixated on winning.
Ali - Speed, both offensively and defensively
FMJ - Savvy
Pac - Speed maybe

 

Duran - Ferocity
SRR - Rhythm
SRL - Versatility
Hagler - Versatility
Joe Louis - Methodical
Jack Johnson - Counter-Punching
Marciano - Slugging
Ali - Footwork
FMJ - Defensiveness
Pac - Aggressiveness

 

D= Ring Savvy
SRR= Power
SRL=Virtuosity
H=Undecided
L=Punching perfection
J=Anticipation
M=Relentlessness
A= Reflex agility
FMJ = Confidence
Pac =Handspeed

 

Duran- Versatility. He could fight inside or out even as a usually shorter man. Very rare quality.
SRR- Hooking off the uppercut and vice versa
SRL- Speed kills
Hagler- Switch hitting
Joe Louis- Calm. Never got overexcited and always picked his punch it's part of what made him such a great finisher
Jack Johnson- Best clinch game in boxing history
Marciano- Endless stamina and toughness
Ali- The strongest mind and will in boxing
FMJ- Fight IQ
Pac- The way he split the jab was sublime

 

Just going with the first thought that comes into my head:

Duran - intensity
SRR - two-fisted power
SRL - ego (in a positive way)
Hagler - versatility
Joe Louis - accuracy
Jack Johnson - physical strength
Marciano - relentlessness
Ali - ring psychology
FMJ - poise
Pac - handspeed

 

Duran: skilled aggression
SRL: speed (hand and foot)
Hagler: complete, well-rounded offensive abilities
Joe Louis: punching ability
Jack Johnson: savvy and strength
Marciano: relentlessness
Ali: speed (hand and foot)
FMJ: ring iq (and reflexes)
Pac: offensive explosiveness

 

Duran - mean.

SRL - cunning.

SRR - natural? (unsure on this, will have a think)

Hagler - I liked what @Jel wrote about fighting like a challenger even as a champion. Would add his siege mentality.

(Louis & Johnson I don't know well enough to really say)

Marciano - determination/will.

Ali - personality/character/mental strength.

FMJr - professional/composure.

Pacquiao - game-ness/aggression (sometimes worked against him but it is what I most associate with Pac)

 

Duran Panama
SRR The coolest
SRL Deadly speed
Hagler Ferocious
J Louis Icon
JJ Misunderstood
Marciano tough
Ali dancing
FMJ Brilliant promoter
Pac warrior

 

Good call on Hagler and the switch hitting!