With over 3.5 billion fans worldwide, football ("soccer") is the most popular sport on the planet. It always ranks at the top of the majority of the credible lists published.
Usually, popularity is measured based on the "number of viewers/fans". According to a website called mostpopularsports.net, here are the top 3:
Football (which is equivalent to American soccer) is the most popular overall
Tennis is the most popular "individual" sport (no team required)
Golf is the most popular "solo" sport (can be played alone).
Additional opinions from WikiAnswers sports fans:
"Most popular" could mean "most watched," "most played," or "most revenue-generating." Based on the variable definition of "most popular" the following observations apply:
The most played sport, not just watched, in the world is also football (American Soccer).
Based on the number and size of stadiums around the world, the number of people who wear football (American Soccer) merchandise, the number of people who are aware of football, and the number of leagues/tournaments worldwide (The World Cup, UEFA Champions League, The South American Cup, The American Cup, The Asian Cup, 70 English league teams, 40 Italian league teams, 40 Spanish league teams) football ranks at the top of the list regardless of the definition of "most popular."
Rugby is very popular in many former English colonies (with the notable exception of the US) and has a large following. However, participation rates fall below that of football.
Manchester United is the most popular sports team and the biggest sport franchise on the planet. It has over 450 million fans all around the world. Its estimated value is $1.9 billion in value, which well exceeds that of the Dallas Cowboys and Real Madrid.