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This is a poorly worded question. There is quite a bit of difference between Mexican and Tex-Mex.

LA and Houston have the two largest Mexican populations and will have the best and most representative regional Mexican food. Houston does have an edge here because:

1) Mexican food is not segregated to one part of the city, as it is in LA. Houston is a working city without zoning, where people live and eat right next to one another. Try to find passable Mexican food in Beverly Hills, on the other hand.

2) Houston has Hugo Ortega (who bests even Rick Bayless if you ask me) and Hugo's, one of the best upscale Mexican restaurants in the country. LA doesn't even come close in this category.

Now, if you are looking for Tex-Mex, it's a completely different story. Houston and San Antonio have both originated very distinctive brands of Tex-Mex food, and I have trouble choosing one over the other.

Skirt steak has been cooked around Texas since 1930's, but fajitas in their current incarnation took off in Houston. Today you find them all over the world, but Houston is without a doubt a fajita city.

Similarly, San Antonio have signature dishes that you rarely find faithfully replicated anywhere else. Puffy tacos, cheese enchiladas, thick and doughy flour tortillas. All unique to San Antonio and in a different world when it comes to Tex-Mex.

So to sum up, Houston is the best city for Mexican food, with a tie between Houston and San Antonio for Tex-Mex.

I agree with fivrforfun. I live in Bend and the Mexican food at Pepe's rocks. My friend Marcos has a restaurant called La Rosa that is also a locals favorite.
Some really awesome burritos de carne asada can be found at El Grullense in Salem OR. In fact when I'm there I get a cooler and ice and load up before driving home... yes, that good!

rozilla - there are some good Mexican restaurants in Nashville. Pueblo Viejo and Pueblo Real in Franklin, Las Cazuelas (can't beat the entertainment on Saturday night) or Los Arcos on Nolensville Rd. are all real good.

C) Los Angeles. Taco trucks - 'nuff said.

I'm going to jump on the San Antonio is not real Mexican bandwagon, too. Some of the worst "Mexican" food I've ever had was there. Guess that's why they call it Tex-Mex.