. . . as one who has lambasted Atlanta's Mexican restaurants; however, at least in my experience, Atlanta has gotten better, and I've discovered places that have long been here.
In the past, in general, I've actually disdained East Coast Mexican food. In fact, I used to claim that the worst place in the U.S. to get Mexican food was New York City. That might still be true; in fact, in my experience, at least, there aren't enough Mexicans in NY to make for a decent restaurant. There's tons of other latin foods, of course, but not Mexican food.
But in the 10 years since I've lived here things have progressed positively. Not only have I learned about authenticas along Buford Highway, but numerous restaurants in the in-town area have sprung up with an authentic Mexican cache. Some are more so, like Mi Barrio on MLK. Others suit a particular kind if clubbing crowd, like the Prickly Pear Taqueria. But go during the day and avoid the nightime bullshit. In MIdtown, Senor Patron is brand new, and that had a decidedly Norteno style to its comida, but it's run by Indians (I mean of the subcontinental variety), so who knows? But I hear those guya are from SoCal, so there you go.
Probably the best for real Mexican food in ATL is still on Buford Highway. I'd try Tacos MIchoacan, or Carniceria El Progresso. But if you want to attempt a bridge of the lunch place and dinner time, try Mi Barrio. If you're interested in a clubby-type place, try Prickly Pear or Senor Patron. But the best place for authentic Mexican Food in ATL is probably some Mexican family's house or mine.