You can find plenty of places to get a giant burrito around St. Louis now, but back in the 90's this was the place for me.
I made a weekly pilgrimage for the shredded beef burrito and quesadilla. They had chips and a big variety of salsas, but the burrito and quesadilla were all I needed. I never touched the fish tacos.
Unfortunately they expanded to 3 stores and I guess it was too much. They closed and sold the menu.
There's a place at 170 and Delmar called Flaco's Cocina that pays "homage to the late, lamented Flaco's Tacos". The food and vibe are worth going there--just don't expect that amazing shredded beef burrito. [Update 11/18/2017: Alas, Flacos Cocina closed early this year. :( ]