Tacos should really be a food group unto themselves—and there are plenty of places in Tucson that can fulfill that niche, some better than others. Seis Kitchen and Catering, part of the Mercado San Agustin, does great tacos, as well as their other Mexican food offerings.
Seis, which began life as a food truck, is now a "brick-and-mortar" walk-up window—food can either be taken to go, or consumed on the lovely Mercado patio, as long as its not too hot for you outside. (The owners still operate the food truck, as well as the Mercado location.) According to the menu, the cuisine is inspired by the six culinary regions of Mexico (hence the name "Seis"), but I didn't find their offerings to be significantly different than other semi-upscale tacos around town.
Breakfast is served Monday through Friday until 11 a.m., and on weekends until 1 p.m., and there are lots of offerings. In order to get a rounded experience, we decided to go with one breakfast burrito, one breakfast taco plate, and one breakfast entrée. Breakfast sandwiches are also available. Burritos range from $5 to $7.99 and are hefty. We opted for the "Jefe"—packed with chorizo, roasted green chilies, scrambled eggs, guacamole, potatoes, cheese and chipotle crema. The chorizo, which is made on-site, was delightful. Oftentimes chorizo can be heavy and greasy, but this was perfectly spiced without the grease. The rest of the ingredients were proportionally balanced, although if you ask me, there are never enough green chilies in anything!
The breakfast tacos (we chose chile verde con papas—$6.99), were also excellent—three house-made corn tortillas packed with scrambled eggs and green chilies, topped with sour cream, shredded cheddar and cilantro; and served with crispy potato chunks as well as a side of "smashed beans" (a chunky refried-style pinto bean dish topped with queso fresco). The plato mixto ($7.99, your choice of meat) was also a generous portion of scrambled eggs and cheese, mixed with ham, served with beans and a warm flour tortilla.
Lunch and dinner options are equally impressive at Seis—only one dish disappointed out of all that we tried, and that was the shrimp mojo de ajo taco platter ($11). The concept was excellent—a corn tortilla stuffed with a "chicharron de queso" (crispy fried cheese in the shape of a taco shell), which in turn was stuffed with garlic sautéed shrimp, shredded cabbage, guacamole, cilantro, queso fresco and chipotle crema. The shrimp were well cooked and tender, but the dish lacked any kind of panache—it just fell a bit flat on the flavors. However, the other taco platter, the tacos al pastor ($9), was totally the opposite and had fresh, bold, robust flavors—chile-marinated grilled pork, grilled pineapple, cilantro, red onion and queso fresco.
The chicken taquitos ($6) were delicious, stuffed with shredded chicken and slathered with a generous heap of guacamole and queso fresco; and the poc chuc (marinated grilled chicken) quesadilla ($9) was huge—a thick, delightfully crispy and chewy corn tortilla absolutely packed full with cheese, green chilies, and the shredded chicken (or you can opt for cochinita pibil or birria, also $9, or just chilies and cheese for $7). It was slathered with queso fresco, chipotle crema, cilantro and red onion, and it was excellent. My only other complaint about the whole experience is that we were never offered a choice of which salsa we'd like with our entrées, and the size of the salsa that comes with the entrée is tiny (also, they forgot to include it on several of our choices).
Seis does not serve alcohol, but a few of the other Mercado establishments do, if you've just got to have that beer with your taco—but you should definitely try out the aguas frescas ($3) at Seis. The selection was the same on both visits: horchata, Jamaica and strawberry lime; and you can't go wrong with ANY of them—they were all fantastic. Scoops of locally made Isabella's ice cream ($3, assorted, ever-changing flavors) are available for dessert, if you leave any room—we didn't.