I think football is the richest sport in the word. Reason being it is the most popular among people. there are many sports clubs whose teams play the matches. they offer a lot of money for deserving players, let alone the owners and advertisers cut, which is enormous.
An alternate view: Horse racing is not called "the sport of kings" for nothing. Thoroughbred horse racing is a sport that requires being a millionaire to even get involved in, much less to be a real contender. But by overall size and scope, football is still the most likely answer.
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Football (Soccer to Americans)
With it being the most popular and most played sport in the world it generates far more money than any other sport.