I do not think that there is any question about this--the most popular sport in Texas, at least as a spectator sport, is football. People in Texas have a reputation for caring a great deal about the sport.
Texas has, of course, two NFL teams, the Cowboys and the Texans. The University of Texas has won a national championship relatively recently and Texas Christian University ended this year ranked #2 in the nation. Beyond that, Texas is well known for the passion that people bring to high school football. The book Friday Night Lights, written about Texas high school football, has been made into a movie as well as a popular TV show.