For tex-mex, Mi Cocina is pretty good. There's one location inside the Galleria, and another at Preston & Forest.
http://www.mcrowd.com/micocina.html
I also like Gloria's for "salva-mex": the menu has both Mexican and Salvadoran. There's a location at Beltline & the Tollway.
http://www.gloriasrestaurants.com
ETA: I haven't been there in several years, but I remember Tupinamba being quite good also. Inwood south of 635.
http://www.tupinambarestaurant.com/