For Tex-Mex, the Original Ninfa's on Navigation is my favorite with El Tiempo not far behind. The history behind these two is interesting; you should check them out I'm almost certain that Tx monthly has Gatlin's rated the highest in Houston and in the top 10 in Tx. Salt Lick may be #1. Can't go wrong with any of the Pappa's restaurants.
Sorry to derail, but I love visiting my brother in Houston bc of the food!
Ninfa's is awesome and pretty much put Tex-Mex on the map in late 1970's.