One meal I never buy, and get angry when those around me buy it, is a regular quesadilla. I'm not talking about a regional version (more on that in a bit), but the basic: flour tortilla melted with an American cheese–sometimes cheddar, sometimes Monterey Jack. There is no reason to buy this dish, because it's easier to make than a freakin' sandwich.
Nevertheless, people buy the meal. And the one fast-food quesadilla that those in my universe seem to buy the most is the Del Taco variety. It's absolutely straightforward: a thin layer of cheddar, and green sauce. I would never buy it, but always steal a slice when people do–it's not bad.
But better? El Rincón Chilango's regular flour quesadilla.
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This SanTana fave is best appreciated for its Mexico City-style food, but the owners know even Mexicans sometimes want simple, fast food. So while they do make amazing chilango-style quesadillas from hand-made corn tortillas, quesillo, and featuring flor de calabaza or huitlacoche, they also cook a massive, regular flour tortilla, using Monterey Jack as the cheese.
El Rincón Chilango, 1133 W. 17th St., Santa Ana, (714) 836-5096.