Football is considered to be "king" in Texas. This can be seen on all levels. The Dallas Cowboys are considered to be "America's team," and many Texans cleave to the fortunes of the Cowboys. On a collegiate level, there is little bigger than the Lone Star showdown in University of Texas vs. Texas A & M football matches each year. High school football in Texas can be summarized in three words: "Friday Night Lights." The high school football playoff bracket involves over 200 teams and reads as a schematic conceived by nuclear physicists. Football is part of the "Texan ethos," and most of the state's figures embrace it. When speaking of Gerald Ford, President Johnson, an embodiment of Texas if there ever was one, remarked that Ford had "played too much football without his helmet." Football occupies such a central importance in Texas that it is difficult to see any other sport dislodging it. It is said that "only two sports matter in Texas: football--and spring football."
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.
One word- football. Like it has been stated, Texas is huge on football. As a die-hard Dallas Cowboys fan myself, this is easy to see when I go to a game in Texas (now AT&T... unfortunately) Stadium. The saying "everything is bigger in Texas" definitely holds true here. The fans are loud, proud, and bold.
Of course, it is hard to ignore other sports, such as hockey or basketball. While not as popular as football, these sports are still very popular.
Also, barrel racing, pole bending, and horse showing in general is extremely popular in Texas. The people involved in the horse showing world are very, very serious about it, and many train themselves and their horses as much as professional athletes do.
Hope this helps!