What has been the biggest event in Boxing so far?

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Ali vs Frazier I.
Most who were around then can still recite the exact date of the bout...March 8, 1971.
How many other fights have that unreal, unforgettable aura?

It was the most anticipated sporting event in history and remains that way still.
Ringside seats went for an unheard of $150. Sinatra shot photos at ringside

Both fighters appeared on the covers of Newsweek and Time when those magazines were the media.
"The $5 Million Fighters," Time breathlessly tagged them.

The entire world stopped and watched that night for one hour.
It's doubtful Mayweather and Pacquiao will elicit that type of fevered response.

Ain't this the truth.

[Rexob]

Tyson vs Spinks for me, both undefeated champions at the time, the awesome Tyson demolishing Spinks inside 2 minutes, was something to behold! and his entrance one of the best! Tyson at his best! and a massive boxing event. Edge of the seat stuff without a doubt!

[sucracristo]

in my mind probably leonard-duran, chavez-whitaker, chavez-taylor, hagler-hearns, but
back in the 40's and 50's everyone followed boxing, and the best ALWAYS fought each other,
usually several times. yeah, there is more money today, especially from the tv and sponsor side,
but i don't think the fights are as significant today. louis-shmelling rematch was enormous
and had so many implications leading into WW2, like jesse owens in the olympics. hard to
imagine the robinson, pep, graziano, marciano, etc fights not being at least as important back
then to people as anything today.

[IRLangmaid25]

From a British perspective you can make strong arguments for the Nigel Benn-Chris Eubank rivalry, which had it all with their fierce rivalry they had between them and it drew massive television audiences on ITV and in their second fight up at Super-Middleweight drew 42,000 people to Old Trafford for their rematch, and generated a lot of interest there. To a lesser extent on that front you have Carl Froch and George Groves rematch, yes it drew the biggest postwar crowd here in Britain by fighting at Wembley Stadium. But the biggest problem it didn't quite get enough attention because of it being on a Pay Per View platform but it did sell.

Away from that you can mention...
Sugar Ray Leonard v Marvin Hagler
Oscar De La Hoya v Tito Trinidad
Muhammed Ali v George Foreman
Joe Louis v Max Schemling.

They are all big in their own ways.

[olij999]

Johnson -v- Jefferies but in the last 50 years, definitely Frazier -v- Ali 1.

It's unlikely we will ever see two undefeated ATGs, both with legitimate claims to the HW crown, battling it out to see who is the 'man'.

+1. Frazier v Ali 1 wins hands down for exactly this reason.