The only place in BA that I've ever eaten that even comes close to Tex-Mex (and it's pretty good) is La Fabrica del Taco in Palermo. I haven't actually eaten there in quite some time, but it's run by some Mexican folk and they sell...you guessed it, tacos. Hard and soft, fairly tasty. They even have some good sauces, with a range from Argentino (at least they used to make a joke about it on the bottles) to fairly hot. I think they may have a location in Las Cañitas also.
There's El Salto de Las Ranas on Rodriguez Peña near Santa Fe (across from the Casa Bar). They have a mix of Tex-Mex and a few popularized Mexican dishes. Another place I haven't eaten at in quite some time, not nearly what I consider authentic Mexican meals, being from Texas (where we don't only eat Tex-Mex, BTW), but not bad for here.
There's another place called Taco Box, which is sort of a Tex-Mex fusion place (not sure what it's fused with, or maybe fusion isn't the right word) that has decent food as well.
The only place that, to me, was ever even close to authentic Mexican food was a small place in Pilar across from the Easy/Jumbo shopping mall out there. Argentine-owned but the chef and some of the people who ran the place were Mexican and they imported stuff from Mexico for many of their plates. not exactly in BA city, but certainly the province, but way too far out for a casual trip unless you own a car, and even then it's a good 45-60 minute drive. though given the last few years under Cristina as far as imports go, I doubt they're even still open, or that the quality of their food still be the same.
funny enough, I didn't look at the list posted previously by link - but all three of my items here are on that list. I may even have to look at some of the others.
But as far as I'm concerned, if you are looking for good, authenticate Mexican food here - don't rty too hard and be ready to be disappointed.