Soccer is the most popular sport in the world. So it would be logic that soccer would have the most fans.
Because it is. Billions of fans, hundreds of millions of TV viewers, hundreds of thousands of professional players. No other sport operates on the same scale.
That would be American Football.
Many sports fans are passionate about their sport and their favourite team. So there is no real answer to this question. Traditionally football fans sing at most grounds and support their team for the length of their lives. Football teams around the world have an attachment to the area and cannot be moved. However cricket fans in India are passionate, in usa gridiron and baseball fans are noisy and care for their favourite team, as do ice hockey fans in Canada. So no one sport has all the most passionate fans. Football has the most fans around the world and thus the highets number of passionate fans so if that is what you were asking, which sport has the largest number of passionate fans, then the answer would be football.