People love to bet online on a wide variety of sports, but some sports and some markets are the most popular -- year in, year out. The outcome of the Super Bowl is one of the most wagered events in the world. So you could say that football is the most popular sport to bet on. That said, the Super Bowl is just once per year, and other sports such as tennis and horse racing operate year-round. These sports are really popular amongst daily bettors. In addition, soccer is the biggest sport in the world -- billions are wagered on soccer alone annually.