Cancun is my favorite taqueria. They have the best quesadilla suiza in town. Also their green salsa is so good. Their burritos are really yummy as well. The place looks a little dirty but if you order to go then you'll be just fine.
PROS: Great suizas
CONS: Place looks a little dirty
Excellent carne asada burrito. Not quite on par with the best in the Mission (La Cumbre, La Taqueria, etc), but if you're in the Market/SOMA/Civic area, it's well worth it.