The Ali vs. Frazier fights were of huge significance in the US and to a degree abroad. Louis vs. Scmelling, however, was watched world wide and carried the weight of two continents, two political ideologies, and two peoples. For that reason, I go with Louis vs. Scmelling II.
I wasn't alive but my pops still talks about Rumble in the Jungle as the biggest fight that he has seen. I'll go with that.