Not sure if it's a restaurant in the traditional sense...but when I visit Miami I have to stop by T-Mex. My only beef (no pun intended) is that there are bones in the chicken sometimes. Since everything is done on site. One of my boys chipped a tooth and I've found one at least 2 times in 10 or so visits (ya, I'm in Miami like that)
There's a Mr. Taco and place called Mercadito too. Marcadito is a littler higher end.