For originality, flavor and texture, it's the chile en nogada taco at a nameless street stall on Calle Tlaxcala (directly across from the IMSSS office at #159, near Metro Chilpancingo in Colonia Condesa).
This taco is made with jalapeño chilies (cleaned of all seeds, so they're usually not picante), stuffed with shredded pork which has been spiced and sweetened, and topped with a classic nogada (nut and cream sauce), then sprinkled with pomegranate seeds. This famous dish, usually made with chile poblano, is found in restaurants around Independence Day in September--here it's available year round. I've eaten many times at this stall, with many friends, and always gone away happy. You can have it wrapped in a tortilla as a taco, or on a plate with rice. The other tacos de guisado choices here are also very good. They open by 9am and close when the food runs out (usually by mid-afternoon).
Be sure to explore the whole block around metro Chilpancingo, and across Insurgentes. It's one of the best street stall areas in town.
NOTE (August 2012): Since I wrote this post I've been back many times. But a few months ago I went and noticed the spot was empty, and nobody knew where they had gone.