There are several biggest internet providers in the USA depending on the type of internet they are providing.
For the cable internet the biggest cable internet provider in the USA as of now is Comcast Internet service who has around 14 million customers in 39 states.
Verizon internet would be the biggest internet provider in the USA when it comes to DSL, Fiber Optic Internet and Wireless Internet service.
Centurylink is another big internet provider who provides DSL and fiber optic internet as well as telephone service in a lot of states.
Centurylink doesn't serve every place with Internet or phone but they are available in most states and towns.