It depends on what the original question defines as "importance". For me it's Louis- Schmeling by a distance, especially from a wider view and a political standpoint. Both FOTC 1 (Johnson-Jeffries) & FOTC 2 (Frazier-Ali) have political significance, but the fact that WW2 was 3 years around the corner, and with all that was going on in Europe, with Hitler and his "Ayryan supremacy" policies, the fight was a global event on both a boxing and non-boxing level. It was billed as a battle of freedom and democracy versus tyranny; good (Louis) versus evil (Schmeling); the whole world seemed to be literally watching - and the fight had the added spice and massive irony that Louis was representing the "free world" and a country where forms of apartheid were still very much in practice. It is difficult to imagine, some 80 years on, just how huge an event this was without reading up on the subject extensively. Louis went on to become a huge symbol pf the US war effort - how sad that Uncle Sam took his money, all because 2 cheques he received for title defences Joe made where he donated his purses to the Army & Navy rlief funds were made out to Joe himself and he had to pay tax on them! That's how Joe's financial troubles started... No doubt Johnson & Jefries was huge too, with the black-white angle of the time; Ali-Frazier had political signifance too & from the boxing perspective was significant as the first HW title fight between 2 undefeated champions...but neither scaled the heights on a truly world wide level as Louis-Schmeling 2. All imo of course...
I think you guys are underated a number of Jack Dempsey fights that could be important. His defense in 1921 made the fight game a million $. Than the Firpo, Sharkey and Tunney fights each drew over a million $, which was un heard of at the time and would not be repeated until Louis Baer. And the coming of Ali.
Louis/Schelming 2. It was more than boxing to the greatest generation. It was a scary time. Joe Louis actually boosted a nation`s confidence and for that he will always be that Dude...the greatest man who ever lived.
Too many to say just one. A lot of them transfused with political, social, and historical significances. I would narrow it down to the two fight series between Louis & Schmeling as the #1. The pressure that those two men must have felt with the weight of their own nations on their backs cannot be measured. Not just Joe but Schmeling too. Johnson vs Jeffries #2 and Ali vs Frazier FOTC #3 What about Langford vs Johnson?