While there isn't a single definitive answer to the "hardest college in Texas to get into,"
Rice University consistently ranks as one of the most selective. Its extremely low acceptance rate, high average SAT/ACT scores, and rigorous application process make it a very challenging institution to gain admission to. Other highly selective schools in Texas include the University of Texas at Austin (especially for in-state students), and some of the smaller, highly-ranked private colleges.
It's important to remember that "hardest" is subjective and depends on the individual applicant's profile. A school might be difficult for one applicant but easier for another with a different academic background and extracurricular achievements.