If it's Friday night and I'm in Dallas, you'll find me at Ojeda's, with the best Tex-Mex in the city. Everywhere else is just disappointing after eating at Ojeda's.
Try the brisket tacos, and of course the margaritas. The sour cream chicken enchiladas are excellent. I also like the puffed tacos (a corn tortilla fried and puffed up, then stuffed with meat, tomatoes, lettuce, etc.) and the chicken flautas. If the salsa on the table is too hot for you, ask for some mild salsa.
During the winter, go for the tortilla soup.