We have yet to find an absolute favorite here (we're from CA and the food is so different there), but these are some we like quite a bit...
Joe T Garcia's (stockyards)
Esperanza's (Hemphill)
La Playa Maya (Main near stockyards, and on Camp Bowie)
Los Molcajetes <SP?> (Beach & Western Center)
Oscar's (377 in Haltom City)
Cristina's (Flower Mound or Southlake)
Angelina's (Lewisville, I think)
Alvarado's (a hole in the wall with drive-thru that has the best shredded beef tacos and cheese enchiladas. Davis & 820 in NRH looks like an old Taco Bell. They are 24hrs too!).