Ask that question in 1975 and nearly everybody would have told you Muhammed Ali without hesitation. Then the movie "Rocky" came out the following year and boxing reached it's peak of popularity.
Howard Cosell, a very popular sportscaster from the 1970s and arguably the greatest sportscaster of all time, championed the sport and helped bring it into widespread popularity to where even housewives were watching it.
Towards the end of his career, Cosell would become disgusted with boxing and vowed to have nothing to do with it and as a result, boxing went into decline. Then the antics of Mike Tyson put a final nail in the coffin.
The Klitschos average about 50,000 people at their fights, plus the worldwide tv audience. ESPN had a hissy fit with them a few years ago and refused to show their matches. Wladimir makes more money in one fight than the MMA makes in a year,that is not a sport in decline.