The best by far is Aparicio's Restaurante and Tequila bar. There food is amoung the best in DFW and nice digs since they upgraded from the "tortilla factory" into their new building.
They are located on 18th and 5.
If you are looking for something even more authentic La Paloma taqueria this is on 5 prety much directly across from city hall.
For more mass market Nuevo Leon is pretty good.