Football (soccer) is by far the most popular sport on the planet. The game is played by over 250 million players in over 200 countries. The FIFA World Cup - the pinnacle of the sport - is the most watched sporting event around the globe; and it has been estimated that 715.1 million people watched the final of the 2006 World Cup in Germany.
Almost all countries in South America consider football to be their national sport and the game is by far the most popular sport in the continent. Guyana is the only exception to this, as cricket is the country's national and most popular game.
Football is the national sport in the majority of western European countries. In Eastern Europe ice hockey and basketball are major sports, although football is still immensely popular here and commands a large following.
South East Asia
Countries in this region have historically not been very strong at the sport, but the game is followed religiously here. For example the national teams of South Korea and Japan have improved dramatically over the past 20 years and generally qualify for major tournaments.
The Middle East
Football is the national sport of the majority of countries in the middle east such as Saudi Arabia, the UAE, Iran, Iraq, Israel, oman, Jordan etc.
Again, pretty much every African country regards football as its national sport, with the exception of South Africa, where cricket and rugby share the honor.
Notable exceptions and their actual national sport
Sri Lanka (cricket)
West Indies (cricket)
South Africa (rugby and cricket)