That's easy, Football Superstars is the best one I've played.
It's an MMO so you get to play against people all over the world. They have a World Cup tournament and the graphics are superb. They recently released version 3 which means you can have Puma/Reebok branded appararel. Its an awesome game, plus it doesn't end there as you get to go shopping, go to the gym, and pick up in the virtual nightclub!
Its free but you must register.
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There are many popular Basketball games that are offered on a number of websites. Games such as Basketballs, Basketballs Level Pack, Stick Basketball, and Basketball Rally are popular online basketball games.
There are many football games one could play online. Football Superstars which will allow one to play in a virtual world and Online Football Manager are both popular with fans.
The most popular football software is probably the Madden series of football Video Games from EA Sports.