Funnily enough I considered posting about the great man but decided not too as i don't think my boxing rants have always gone down too well .
He was an incredible fighter and in many ways underrated. When you look at his record his defeats or draws were all contentious decisions (Hagler often the victim of being the away fighter). He was never stopped and thanks to the politics of the day a man who only got a title shot in his 50th fight. When Joe Frazier was asked why Hagler couldn't get big fights he answered with "You have three strikes against you, "You're black, you're a southpaw, and you're good.". He ruined Minter who pissed off Marvin with a racist remark "I'll never lose to a black man" that Minter later said was a misunderstanding (only Minter ultimately would know the truth of that). Then you have landmarks in the sport like the eight minutes and one second against Tommy Hearns and the brutal 11th round victory against the Ugandan powerhouse John Mugabi.
There are two fights I'd point people towards that are lesser known. The second of the two fights against Mustafa Hamsho where you can see Hagler step on the gas and really go for him after being repeatedly fouled. It's a masterclass in suddenly going for the knockout, and a fight against the British Champion Tony Sibson. Sibson has a genuine go and it's a perfect demonstration of an elite level fighter in Hagler against a very good level fighter. Sibson himself would say as much post fight.