Actually, Fajita's are the textbook example of Tex-Mex. Food invented in Texas or seasoned with a more chili-like flavor, but with a Mexican Flair. Most American Mexican restaurant (El Chico, Chi-Chi's, etc...) chain's are actually serving a more Tex-Mex menu. True mexican food is substantially more bland than what most people are used to.
With that said, If you really want to set your mouth on fire, try some New Mexican (the state) food.