What type of tortilla is most often used for a soft taco in northern Mexico?

The tortilla most often associated with a soft taco in northern Mexico is a flour tortilla.

Wheat flour tortillas are especially characteristic of northern Mexico, where wheat cultivation and regional food traditions helped make them a daily staple. They are soft, pliable, and sturdy, qualities that suit tacos filled with grilled beef, carne asada, machaca, or other northern Mexican specialties. Flour tortillas are usually prepared with wheat flour, water, salt, and fat such as lard or oil; some recipes also use baking powder.

This answer describes a regional pattern, not an absolute rule. Corn tortillas remain central to Mexican cuisine and are widely used for soft tacos throughout much of the country. In fact, traditional soft tacos can be made with corn tortillas, while flour-tortilla soft tacos became especially familiar through northern Mexican cooking and commercial restaurant versions.

Flour tortillas also help distinguish some neighbouring dishes. Burritos typically use a larger flour tortilla that is rolled closed around its filling, whereas a soft taco is commonly smaller and folded rather than sealed.