There is an unassuming sea-side stand near the town of San Blas on Playa Hermosa, just north of Puerto Vallarta, that has the best Mexican food I've ever had. Fish tacos, burritos, enchilada's and very cold beer. All for $1US each. Spent 11 days there hanging out in a hammock on the beach during my motorcycle ride from San Diego to Buenos Aires. I almost married the cook's daughter. Such a dark-skinned beauty ... with eyes like emerald pools .... long, straight black hair ... brilliant smile ... oh, sorry.
We had Mexican last night at Mole on Cabildo 1400 (mas o menos) in Belgrano. Tried it 4 years ago and it was horrible. Last night was pretty good. However the "spicey" Mole Chicken that they had rated at "3 Peppers" didn't really hit the hot scale. You need to ask for their "special" hot sauce if you want anything with zing.
I have been routinely disappointed with Mexican food in Buenos Aires. For Ameri/Mex you can try California Burrito Company in el centro. Maybe it's on Lavallle?