Los Cucos, the fajitas a la diabla are excellent. There are dozens of good places here in San Angelo. I don't recall ever having bad mexican food here.
I spent almost every single lunch hour both my Sophmore and junior year at Rosa's Cafe. I still go there every time I get to San Angelo.
Mejor Que Nada used to be good as well. Last time I went (3yrs now) they weren't as good as I remembered.
Rosa's is great, they have some pretty cheap taco plates on Tuesdays. Mejor Que Nada went downhill a few years ago and ended up closing down. There is a home cookin' style restaurant there now called Paula Sue's. Mejor had very good food in its glory days.
If your in San Angelo and want some great Mexican food, I would say the best is Franco's at MLK and 22nd St. It is a little hole in the wall, but the service is great and the food is outstanding. This place is always packed, not a lot of seating, but the food is fast and hot when it arrives.