If you're in Oaxaca, my single strongest recommendation is to visit Cenaduría Doña Gloria around 9…read morePM and order a tlayuda with tasajo. When you receive the tlayuda, ask for the onions (cebollas), which Doña Gloria will hand you in a Tupperware container. Relax, eat, and stare at the stars in her front yard.
I tried 5 or 6 other tlayudas in Oaxaca, including much better-known ones like Tlayudas Libres, La Biznaga, and it's not even a contest. Near the end of my trip, I brought a guest here who was in town for a week and he'd had 2 other tlayudas. Visiting Doña Gloria restored his faith in the dish.
Her cenaduría opens about 8:45 PM every night and stays open past 11. I posted a photo of the sidewalk sign you'll see on Rio Amazonas, midway between Rio Grijalva and Rio Consulado. Yelp's map placement is accurate within 20 meters. She's well known in the La Cascada neighborhood.
I posted a little more about my experience here: http://troy.yort.com/oaxaca-street-food-recommendations/.