For my wife and me, the best Tex-Mex in San Antonio was Los Barrios, followed closely by Mi Tierra. Admittedly, this wasn't a scientific or comprehensive sampling as were there only for a few days, but those are the two that we came to like most.
At Los Barrios, they have a thing called a "quesataco" that comes in a slightly thicker tortilla and has bacon as well as whatever other meat you order (i.e., chicken or beef). I had the chicken quesataco on our first visit and it knocked me out. I thought about it so much that we went back for another before leaving.
At Mi Tierra, they have a roast pork bit dish that was also a grabber for me. To take a flour tortilla, slather it with some avocado, add some hot sauce, some lettuce, tomatoes and beans, then fill it with the pork bits was pure heaven.
We also had dinner at Casa Rio on the Riverwalk, and, while we enjoyed that dinner, we liked the other two places better.
We had a great time and really enjoyed San Antonio. However, it's time for them to update the Alamo movie. Even John Wayne would be a little embarrassed at its jingoistic tone.