Check this website out as a restaurant guide tool. And Napoleon is again right, spicy food is not too common here. And do not dream of chimichangas, fish tacos, taquitos either - sour cream is also hard to come by. Nachos or carne asada burrito and mexican rice real stuff it is best to do as I do, cook it yourself. The ingredients would take sometime to hunt for but if you can make it, I am sure it would be the best. Jalapenos are not too common here either. Boy, what I would give for a a sopapilla or ceviche right now. One note, do not go to that RANA something place - it was a waste of money for not-so-Mexican-food! Good Luck hannstew.
I also share this craving for fiery - authentic Mexican cuisine. I have visited the fore-mentioned spots in Palermo, and for what it's worth, they are not that bad.
Not to say they are any where near authentic, or picante. The best bet is, in fact, go on the hunt for ingredients and prepare it yourself. Argentina has a depressing way of taking the cuisines of other cultures, twisting, manipulating, and de-seasoning the hell out of them until they no longer resemble their original inspiration.
As a chef, I never have too much trouble rooting out the ingredients and satisfying a craving (although some things are truly impossible to find). On a related note, however, it has become very dificult for me to dine out without being over-critical. I just started working for an "Irish Restaurant", and I am struggling to influence the owners to actually serve food that is traditional Irish fare. When was the last time you heard of gnochi, milanesa, and flan being of Irish influence?? I thought not. Folks down here are proud and stubborn, and indeed, "want what they want". God forbid a chef be allowed freedom of exspression without going out of business.
Sorry about the tangent....just my two c's !!!!!